Do you foul when up 3?

It is something I have wondered forever, if you have the lead in a basketball game by 3 points and time is running out, why doesn't the team winning foul the other team not allowing them to shoot a 3-pointer to tie the game? Then the other team gets two free throws and cannot tie the game. It forces the other team to have to make the first free throw, attempt to miss the second free throw, get the rebound and score. It really hit home when Georgia was playing Florida a few weeks back, in OT Georgia was up three and Florida ran the ball up the court and shot a long three, making it, tying the game. Florida ended up winning in double OT. Mark Fox, Georgia coach, was asked about it and said that he instructed them to foul, but the message did not get heard, obviously. Not to mention Georgia had a timeout, might have wanted to use it so you could make sure they understood to foul. There is no doubt in my mind that you foul in that situation! This happened again when Georgia was playing South Carolina. USC was down by 20 points and made a run to get within 3, Georgia chose not to foul again, this time it worked out with Georgia blocking the three point attempt and holding on to win the game.