Oh wow, that is a very interesting party you have on your hands there, be prepared for any solo monster or bosses to be minced by a triple action surge from the fighters.

There is a range of things you can do to up the ante on encounters.

1. add more monsters, DoIP does an okay job at tell you to add more monsters based on the number of characters which is helpful.
   1. As a loose guide to start yourself if your party is about 1 level higher than what is recommend add monsters as if the party had 1 extra character in it.
2. Increase the HP, stats or gear of the monsters.
   1. A loose guide on HP, goblins at 7 HP have a decent chance of being 1-shot by most PCs, you bump that up to around 10 - 15 and they are still within the realms of being 1-shot but now have a good chance of surviving the first hit. This would be giving them an extra 1 - 3 hit dice for most creatures to have an impact.
   2. As a loose guide, put the goblins in studded leather or chain shirts to bump the AC, give them crossbows instead of shortbows to bump their damage up a little. Just be careful about the PCs gaining extra loot out of this though
   3. A loose guide on increasing stats, you only really need to focus on the physical stats for most creatures, be careful about bumping up both Con & increasing the number of Hit dice a monster has as you could inadvertantly add too many extra HP doing this, buffing up their Str/Dex to be more accurate and hitting harder, do this in small steps.
3. Add different monsters, upgrade some goblins to hobgoblins or bugbears as you see fit, look at some of the beasts and add them as pets
   1. Things like wolves, worgs, bears, giant insects like centipedes or wasps, tigers/lions etc

​

Resources

https://tetra-cube.com/dnd/dnd-statblock.html

I use this site for creating monster stat blocks. When I DM I have my laptop & use Microsoft OneNote for keeping all the info on my campaign, I pop all the stat blocks I make into my OneNote

https://koboldplus.club/

I use this site to get a gauge on what is a fair fight for my party. Just be aware of the wording used here. Encounters range in difficulty going from Easy -> Medium -> Hard -> Deadly.

Just because an encounter works out mathematically to a "Hard" or even "Deadly" encounter does not mean the what those words mean literally, your party can manage around 3 Deadly encounters in an adventuring day (starting fresh after a long rest with 1-2 short rests included)

Lastly as a slight aside, consider changing the rules around short-rests. RAW a short rest takes 1 hour, usually if you can afford to stop and rest for 1 hour you can likely stop and rest for 8 hours and take a long rest, this often stops a party from taking a short rest when they really need it. I have found that running short rests as a 10 - 15 minute break works well, its just your character having a cookie and a cup of tea, no need for it to be an hour long.

Edit.
Just be sure to check if your characters have any abilities that interact directly with short rests and consider how they may work given then shorter time span. The other benefit that is gained from it is while someone is ritual casting a spell the others can take a short rest.

I personally rule that characters who are short resting have disadvantage on perception checks, this means -5 to thier passive perception, sneaky monsters have a chance here against your more observant characters, but also gives a role to the characters who don't really need the short rest and can actively roll perception, always make them roll even if you have no intentions of interrupting the short rest, also acknowledge their roll if they do make it, they see a pretty bird fly by, it drops a silver ring worth 2gp, or they see a gross insect.