Posts from All castles in Heroes 3.
But at least as a first approximation.

Справочник по НоММ-4 http://handbookhmm.ru/pamyatka-dlya-heroes-iv.html
Памятка по НоММ-3 http://handbookhmm.ru/pamyatka-dlya-homm-iii.html
и HotA http://handbookhmm.ru/vyishla-obnovlennaya-pamyatka-dlya-homm-iii-hota.html
What is this utility, and where can I get it? Also, does it affect the attack power of an army, or just the army's size in hit points?
Справочник по НоММ-4 http://handbookhmm.ru/pamyatka-dlya-heroes-iv.html
Памятка по НоММ-3 http://handbookhmm.ru/pamyatka-dlya-homm-iii.html
и HotA http://handbookhmm.ru/vyishla-obnovlennaya-pamyatka-dlya-homm-iii-hota.html
I looked, but couldn't find it. Please provide a link.
And still – is army strength determined by attack power or by the number of units (hit points)?
I already mentioned that in the FизМиГ, comparative results of such calculations are provided, which means they offer some value, as they are used to characterize Castles. The issue is that there are no absolute results, only relative ones, and only for attack, not for defense...