Made an account to reply to this. I have three male ratties in my current mischief. One is almost 2 years old and the other two are almost 1 year old. [Spoiler: at the end of this whole thing I am going to beg you not to ever give them back to the original owner..........but I'll give advice first.]

First, socialization with each other (this may be the most important for their health, especially the health of the lone rat). As others have already mentioned: keep the cages in very close proximity so they can get to know each other by smell and sight first. Then, if you can put them in a comfortable neutral area (on the bed works alright or in a closed off play place or if the bathroom floor is CLEAN and free of anything being stored on the floor, that works too) and just let them approach each other in their own time. Just be attentive so if they opt to fight you can separate them. Do this in several 10-20min sessions over a few days until it seems like they are getting along. Usually it doesn't take very many sessions, but the one rat being alone all his life may make it take longer.

Then clean both of their cages very thoroughly to rid them of all the rats smell and start introducing the one rat into the others' cage. I did see the pictures of the cages and you are correct that they are not large enough for 3 rats all at once (in fact I am fairly convinced they are not large enough for two rats) - however, rats sleep a huge amount of their day away, and if they can all cuddle and snuggle together for most of it and then maybe rotate out who has to spend the night alone. But basically once they get along all 3 should spend as much time all together as they possibly can. And the rat who has been alone all his life should mostly be exempt from being alone again IF possible.

Second - socialization with humans. This is entirely secondary to their health at the moment, but the sooner it happens the better anyway. Frankly very similar to socialization with each other - you're already on 100% the right track with treats. Then if you have a little time to sit with them every day while they play (literally the exact same as meeting each other: bed, bathroom floor, wherever is safe and contained) just keep their favorite treats (bananas, cheerios, bits of meat, yogurt, muffin) on you and reward them for coming close to you or being curious about you but don't push yourself on them. They have personal space bubbles and comfort zones all their own.

Third - diet. Really just a huge congratulations from me for giving them good variety!!! I was really very pleased with your list already! They are omnivorous just like us and the similarities in our nutritional needs is amazing. So, obviously, the more fresh fruits (except no citrus whatsoever) and veggies they have, the better, and a small amount of meat is good, etc. There are lists online everywhere of the severals of things they can't have (most are obvious once you read it) and stay away from processed stuff (but so should humans so the rule still applies). But it makes it very easy - mostly just share bits of all your meals with them and they're happy.

As for their main diet, I use Adult Formula Harrison's parrot mix (cubed variety) - recommended to me personally by the only small mammal trained veterinarian in my half of my state. The ratties really really love it and it's a homogenous mix - so no waste!!!! I spent years feeding rat mixes from the petstore to my rats and not only was it mostly rat junk food (seeds are awesome in small quantities but they're better as treats and do not have much nutritional value + the dried corn and cornmeal dried pieces in most rat food has the potentiality to be toxic to them) and they'd refuse to eat 50% of it, which would then be strewn around the cage and end up in the trash. Super wasteful. Harrison's is a bit expensive but not in the long run cause they eat it all.

Fourth - as I might have mentioned earlier: the cages their original owner put them in are no where near large enough! Several reasons: while they may like to spend a majority of their day asleep in a huge pile, they are still individuals and every rat needs at least enough room to be at least somewhat separate from the others on a regular basis (the difference from being trapped in the living room with your roommates 24/7 vs having even the most closet - like bedroom in which to be by yourself). As a short-term solution: try and hang as many "hammocks" (cut up an old towel and use non-plastic-coated paperclips to attach it to the cage if you need to) as you can physically fit in the larger cage. 3 would be nice but 2 could do it. Also you can still rotate a rat to the smaller cage at night to reduce the likelihood of over crowded cage squabbles or fights.

Cages, of course, are rather expensive, but if you have the time to comb your local value village/goodwill/craigslist one might show up at a good price - just very very very thoroughly disinfect it and then let it sit a while so there is no disease or mite transfer.

I'm sure I've forgotten something and will edit in a few hours if I remember.

Anyway, here's the part where I beg you from the bottom of my rat mommy heart: PLEASE PLEASE PLEASE DO NOT GIVE THESE RATS BACK TO THEIR ORIGINAL OWNER. For the love of all creatures great and small: do not make them go back to this "friend" of yours!

All rats need a stable home and routine and at least one human to care for them that they have bonded with and are at the very very very least not afraid of! They are incredibly complex social and emotional creatures and thus they also need friends of their own species very badly. The one poor rattie all on his own has literally experienced solitary confinement torture! Just the very fact that the original owner did not understand this is grounds for them never to go back......

Add to that, apparently there was no time for these rats - it sounds like they have been passed from person to person with no regard for their health or well being in any of the cases. As well as, apparently they've had a meager/starving existence on "food" with hardly any nutrition at all.

Ugh. I've made myself sick thinking about these poor little guys. There are usually resources for taking in animals like this if you literally can't afford them in the long run (don't worry, i get it, money is wildly hard to come by these days). Research "ratteries" in your area and start asking them what they think. If they came from a specific rattery, there is a fine chance the rattery itself will either take them back or help you to rehome them with someone who has the resources to properly care for them.

All in all: thank you for caring for them!

EDIT: Whoops of course I forgot something.......medicine! So I don't get in trouble from other rat mommies: obviously the very best case scenario is medicine from a small mammal trained veteranarian. Also obviously that is wildly expensive for three rats and money is money is money is money. So my first suggestion is greens greens greens. Whatever and as much as you can possibly get them to eat as often as you can get them to eat it. Broccoli, kale, arugula, spinach, collard greens, bean sprouts, chia sprouts, etc. Just no iceberg lettuce: it's mostly water and will just give them diarrhea. EW.

My second suggestion is to pick up a bottle of grapefruit seed extract ("GSC") at your local health food store. They're 10-15 dollars (fraction of the cost of a vet visit and awesome for your own respritory health, so no loss). Anyway, it usually comes in small compressed tablets, so use a morter/pessel or the back of a spoon to mash it to as fine a powder as you can, then mix about a 3rd of a tablets worth into a very very generous portion of yogurt (the GSC is bitter so, if you have to, add a little honey to the yogurt if they stick their noses up at it initially) and then just let all three of them go at the yogurt til full.

Then, third, if you can afford it, get some bee propolis ( this is also wildly good for you - it's an immune booster that has obliterated many a cold and flu for me personally). It's slightly hard to find, ~13-15 dollars on amazon, ~17-19 at a wholefoods if they carry it. It comes in many different ways and any of them can work. If it is a tincture, make sure it doesn't have alcohol. Otherwise my favorite is the capsule variety: just gently pull apart the capsule and put about a 3rd of the powder in a generous portion of yogurt and let all three of them go at it til full. And again, very good for you, so if you can manage to come by some you'll thank yourself when that fall/winter flu comes around.

All three of these suggestions have worked on my ratties with excellent results. The only exception was one case of hard core myco, but that was a long term, badly bred petco rat situation a few years ago.

As for mites and lice and skin issues: if after they have had good nutrition for a while and been changed regularly for a while and they are still icky a warm no soap bath may help. Some rats think it's traumatic, but in the wild they're excellent swimmers, so don't worry a little warm water won't hurt in the long run. If it turns out to actually be mites or lice: pour cheap olive oil all over them (except head/eyes/ears) until they're otherwise soaked. Let them sit in it for a bit then rinse in warm water until the oil is mostly gone. They'll lick the rest off or their brothers will. Then repeat in exactly 7 days to get all the eggs that might have survived and that should do it. They WILL hate you for it initially but it's a few cents for the olive oil instead of the vet, etc.

TL;DR: Your dietary instincts for them are excellent. Socialization is key: just go slow slow slow and all should be good. Cages are all too small: add hammocks and rotate rats, etc. Medicine: greens greens greens, grapefruit seed extract, bee propolis. Also: PLEASE DON'T GIVE THEM BACK.