We don't really check as the goal of the jam is to help you make a game. You can submit even if it took you the whole weekend to make your submission! (When submitting, there is a field asking how long it took you)
Another reason we don't check is because there isn't really an incentive to lie about it, there's no prizes (or judges, everyone who participated can vote). The only "reward" for making your game in 3 hours or less, as far as we are concerned, is eligibility to enter the Hall of Fame. But the fact that you made a game is, IMO, a better reward :)