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

That's understandable. But you need to enter it as --.--.--. Since I've never used it, I don't know what that means. And why are there 3 of them?

Just to remind you, there are 3 parameters in a chess timer (Start, Move, Battle):
Start - the initial time given at the beginning of the game.
Move - the time allowed for each move. If you spend more time on a move, the excess time is deducted from the Start time. Conversely, saved time is added to the Start time.
Battle - the time for EACH battle. If you exceed this time in any battle, time is deducted from the first parameter, Start. The reverse does not apply (i.e., if you complete a battle much faster than the allotted time, the excess time is simply lost).