Heroes 5.5 WarGame Edition - Tactical Edition.

It's also inconvenient to have it every 3 days, as it forces the player to remember which day and with which hero they already used this bonus. And, what's worse, to keep this extra information in mind between turns.

Actually, no, the spell will simply be marked in gray. Although, if we can't figure out how to display the cooldown, and there are many heroes, then yes, it will be inconvenient.
It could also cost mana. Ultimately, you decide which implementation best suits the idea of the mod, and my job is to offer suggestions.