I don’t have many friends who enjoy Sentinels, so I mostly play Solitaire games with myself. I play three heroes, because that’s really about all I have the physical room for unless I’m willing to set up a table and walk from chair to chair, which I’m not. I only keep records of maybe ten games at a time, so I can’t really confirm my history here, but I’d guess I lose about as many games as I win.
Yesterday I played two games with a friend. We each controlled two heroes, and we absolutely mudstomped the villains.
Now, there are a lot of variables at play when determining my record, obviously. The largest part of it has probably been how I use dice to randomly determine which decks I play, though yesterday I couldn’t help noticing all the conversations my friend and I had about which one of us would be handling which evil effect was on the board, because we both had options to deal with it. This makes me wonder if four heroes isn’t a better number than three; if the increased number of options available to the group doesn’t offset the pain of having the villain’s and environment’s damage boosted above what can generally be reduced down to one or no damage at all.
I've typically played with two friends and normally one of us will play two heroes. Four heroes seems to be a good balance for a majority of games. In the case of V5 villians it seems to be rough no matter how many heroes/villians you play of it. Though in V5 it always seems best to take out Ermine first if she is part of it as her deck manipulation is more difficult to deal with as the turns go by.
And certain villains increase their difficulty in other ways based on H. The dreamer is much more difficult on H=4 than on H=3, and H=5 is near impossible, but that's just because you have to survive much longer, getting closer to Pydro's number. Miss Information is the same way, she requires you to take more bad cards, and she deals more instances before you're able to hit her.
That said, almost every other villain is easier on H=5. Voss is a great example of this. He scales with H by having more starting minions and his damage on his back side. The minions deal more instances of damage. This is why if the heroes can get past his DR, it's almost always better to leave him on his front side and beat on him there, where after you're down to 1 his damage is minimal.