2 KostyaN
The army of the third hero is used exactly as you described.
Specific cases:
1. When playing as Necropolis, transferring the army of the third hero into skeletons;
2. On grass: using the fastest units of the third hero to quickly move and equip secondary heroes;
3. Using meat shields (including covering archers) and removing retaliation;
4. Morale when troops of two alignments are together (provided there are no undead) - neutral;
5. Units from the army of the third hero for "leading neutral creatures" under a ballista or ranged units (Gurnisson starts, a swamp creature with snakes comes as the third hero).

But in general, different situations can occur... It depends on who the third hero is, what resources are available, and what secondary skills they have...

I don't know the answer regarding Shakti and Valeska. I don't have any patches, and I've seen Shakti with three stacks. I don't remember about Valeska, though...