It seems to me that if you raise the heroes' starting Conquest by a high enough amount, most quests could become balanced with 3 and even 2 heroes.
The quests of the base game, at least, seem winnable to me if the heroes' starting Conquest, instead of the usual 5, would be something like 10 for three heroes and 20 for two heroes.
The numbers are just pulled off the top of my head so they might be (way?) too high or too low, and it most probably depends on the quest, but couldn't the principle work?
Usually, when we are only 3 or 4 players, we play something else than Descent, but next time I'll suggest to my group we try it out. Especially with 2 heroes, I'm quite curious as to how much additional starting Conquest would be needed to make games balanced.