Auto-translated
I don't want to go into detail about the different tactics (in short, I usually controlled three heroes and a squad; the heroes had skills in Order, Life, and Tactics, and later I added Combat, Logistics, and Wealth). But this is for the Life and Order castle; the upgrades are somewhat different for other castles.
Regarding your comment about Order vs. Chaos: Order has other powerful abilities (hypnosis, forgetfulness, berserk, etc.). It's all about balance.
In terms of battle tactics, the Tier 3 units are clearly inferior; I'll try to explain why.
Take a standard, well-developed hero (see above). In 90% of cases, the battle tactic is to slow down the enemy, wait, shoot, and finish off the survivors (if there's a hero who wants to escape, either wait a little longer or use blindness, expert prayer, etc.). During a siege, the arsenal is slightly more extensive.
If you've played Heroes 4, you'll know that there are even neutral units (comparable in level and numbers to your army) that can't be defeated without using specific formations and magic, or you'll suffer colossal losses (for example, water elementals protected by something quite tough). That's all for now; sorry, I have to run, it's a holiday after all