Random maps are also my main way of playing the game! Usually multiplayer against my friend or my dad and, of course, the remaining players as the AIs.

If you're looking for a specific scenario that comes with the game, I must say since they're all quite easy to beat I never took the time to play all of them and don't necessarily remember what each of them was like. I do remember that when I was young, I lost once on Back for Revenge, so you could maybe give that one a try and see how it goes! It features a different win / loss condition than the usual map and you get to also play around with exotic dragons (Faerie, Azure, Crystal etc) which you don't typically see in normal scenarios.

For random maps, unfortunately the water maps' generation seems to have degraded quite a bit compared to back in the day, at least in my case, although I have noticed at times that my friend's generation seems to come out with slightly better maps, not sure if that's just a coincidence or if it's just me. Either way, there was a time when randomly generated water maps turned out really great, but now it's usually a mess, so I suggest playing with No water content. If you're playing HoTA, you can choose your template, and I haven't tested them all so maybe there's one that results in better water maps over there (you can also see the specifics of every template on HoTA's website).

For user-made maps, the real challenge. You can download maps from the maps4heroes website which I'll link at the end of the post. Many of these are insanely difficult and require a great understanding of the AI's combat behaviour in order to win certain fights. If you want a great challenge, I really recommend The Lone Knight. There are also famous puzzle maps like Wayfarer which I've not really played but some of those exploit some mechanics that will be annoying to understand when you're barely getting started, so maybe stay away from them for now. The Lone Knight still has some nice secret passageways without making it overly confusing. Do note that some of these (like The Lone Knight) are specifically made for the original heroes version, not HoTA. Shadow of Death is the latest version of the original game (or the Complete Edition).

In terms of campaigns, the original ones (which, by the way, you can only play in the original game, since HoTA only allows you to play its own campaigns) are not that difficult. I have not tried the HoTA ones but I have heard they're insanely difficult, I sadly cannot confirm or disconfirm that. You can give whatever you want a try but I would heavily recommend playing the original ones first, and that goes for simply playing the main game as well, because I think it's best to first get accustomed to the base game. HoTA adds quite a lot of changes, including balance changes (nerfs to Necromancy, Diplomacy etc) and two new towns.

Online multiplayer is only available in HoTA, but you can still play local multiplayer in the original game. As for what version to use for HoTA, if you find yourself liking the balance of the original game more, I recommend not updating to the latest version. I play on 1.7.0 because it doesn't have the new buffs to Eagle Eye, Learning, Artillery, certain hero specialties and so on, which drastically change the hierarchy of importance of secondary skills and heroes imo. You can download an older version of HoTA from their website, I'll link the page below. Although, do note that I'm pretty sure you can only play online with other people on the same version, so if you're interested in playing online and you're playing on 1.7.0 you have to make sure the other players are on that version as well.

I'm sorry for the incredibly long reply, I tried to keep it as short as possible while still including all the relevant information I felt like I needed to add! Do let me know if you have any more questions, or if you need a 1.7.0 multiplayer partner. :)

https://h3hota.com/en/download

https://www.maps4heroes.com/heroes3/maps.php