How do I create a custom hero for a scenario?

Hello everyone, this is my first time on the forum! Let's get straight to the point: I want to create a hero for my map with a special (non-standard) specialization (for example, air magic, something like +10% to air magic damage per level). Can you suggest a program (preferably for HoMM3 Complete) that can be used to do this, if one exists? Thanks in advance!