From The Heroes of Might and Magic III wiki

Secondary skills for the Heroes in Heroes of Might and Magic III.

There are 28 secondary skills.

[edit] General

Your hero usually starts with two secondary skills. Those skills give benefits to you or your army according to the skills. Each hero can have a max of 8 secondary skills, and there are three different levels: Basic, Advanced and Expert. Whenever you gain a level up you get to choose between two skills, either upgrading an existing one or choosing a new one, until you have 8 secondary skills at the Expert level.

There are also other ways to gain secondary skills, by finding a Scholar, a Witch Hut, from a University on the world map or from a Magic University (the four magic secondary skills) in a Conflux (only in Armageddon's Blade).