HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

by the way, besides the bug where the number of troops increases by one when hiring them from an external source, there is also a bug where the number of units changes by +/- one when splitting a stack of troops in the exchange window between heroes... it seems to be the same thing, but simply in the hero's window when splitting a stack...