Secondary skills in Heroes 3.

Logistics - as quickly as possible.
Attack - also preferably at the beginning.
Ranged attack - when playing as Castle, Tower, Stronghold, on small maps with Conflux.
Then for larger maps:
Earth magic - Earth magic expert is IMHO better than any other magic expert.
Wisdom - without it, any magic is useless to level up.
The rest depends on the map.