Secondary skills in Heroes 3.

Of course, it's possible! Such a build is viable! For example, when playing as Crag Hack, I would gain an attack skill for each level and only occasionally other skills.

Up to a certain level (I don't remember exactly which one, but no more than 20), yes, but after that, the probabilities become more or less equal. I used to have information where the probabilities for each type of hero were listed, but it's lost somewhere. You can check with FizMig; he might have it.