Secondary skills in Heroes 3.

In my opinion, so much depends on the map and who you are playing against—the AI or a person. Absolutely mandatory skills:
- Wisdom;
- Earth Magic;
- Air Magic;
- Logistics
The others are mostly as follows:
- Intelligence;
- Water Magic (especially if there are many archangels in the army);
- Archery (of course, unless you are playing Inferno or Fortress);
- Pathfinding (if there are many swamps, etc.);
- Diplomacy (obviously, if there are many neutrals of all levels set to aggressive);
- and Sorcery (if you are playing Necropolis).
Although I played one map where you can't do without Keen Sight; by the way, it's called First Blood.

Generally, in most cases, the skills are: Logistics, Earth Magic, Wisdom, Archery, Air Magic, Intelligence, Water Magic, and Pathfinding.