I'm trying to add new maps to the game, but I'm having problems.

Also, the maps might be in the HotA version or some other mod, like "The Legend of the Red Dragon." Or the maps might be in ZIP archives. If there were screenshots, we could give a more precise answer. We don't know what version you have or where you downloaded it from; there are many variations. Maybe you have Heroes3 HD – this is a re-release of "The Restoration of Erathia" from 2015? Then it won't recognize maps from "Armageddon's Blade," "The Shadow of Death," and especially WoG and HotA: they all have the same extension, but the version codes are different.