Champs need to be generals until later stages of the game.
I would not go this far. All three groupings are viable at all points of the game (tank + mage hero pair, hero + army, solo hero)
My guideline is 'the smallest force necessary to take the enemy without risk".
After all, when you have just two heroes together, no experience is lost, and sometimes, some is gained. That is, the total experience is shared between two heroes, but if they have Trainer II or III, the way Procipinee and Alisa both had in my last play-through, the total experience given to heroes is actually higher than if one of them was soloing.
Of course, once my Empire grew, and there was a lot of work to do, I split them - Procipinee (the mage) kept the army, and Alisa (the tank) went solo.
