It's designed so you could pick any time you have over the weekend (it doesn't have to be 3 hours straight). The idea is - instead of watching a movie, you could make a game and do something more productive to improve your skills!
Regarding the "proof", you are responsible for the time, no one will check it. You will have to put it down when submitting the game. There is no reason not to be fair, since the whole point of the jam is to get better at making games, not to win prizes :)
Hope it helps!