How to arrange 3 heroes?

You can combine any 3 heros in an APC formation. If you have the hero slots unlocked through research. The only thing to check is a few heros have a skill that relates to one troop type, and will benefit more if they are aligned with that troop type, E.G. they may give more bonuses when leading vehicles etc.

You may also consider the level of skills in each hero and hor may troops they can add to the formations and decide which slot you want them in.