GF and I just started the new shadows of nerhkal campaign and we decided to take it down to three heroes (1v1 players) just to speed up the game a bit and so we didn't have an overload of abilities to remember. I'm playing heroes this time around and the intro quest came down to the absolute wire.
She had 3 citizens stolen and would have consumed the fourth for the win on her turn. My apothecary and zerker were both down and her lieutenant (my win condition) was far away and around a corner, but at 2 health remaining. I had to move and stamina push, giving myself a single attack. I rolled a 4 damage attack, no surges, and she rolled a 2 on defense which gave me the win. Even then, I only really got out with 25 extra gold from what the quest would normally give on an overlord win (she pulled 3 threat from the quest as well so... yea...).
Point being for now: yes it was the first quest, but it was crazy close even though were are playing with 3 heroes. Our rolls were pretty even. I had a few misses and a LOT of blank defense rolls, but she missed a couple times with her master ironbound. I'll have to see how the rest of the campaign goes but it doesn't seem unbalanced. I felt like I had to really push for a good strategy to win, and was pretty worried when I missed a few important shots.
I think 3 heroes is just fine, but agree that the best situation is 5 players running 4 heroes and an OL. Don't be afraid to run it with any amount though. There is enough room to make mistakes, and also dice are heartless bastards...