In a seven-player game, with 60/120 blinds, you (3500 chips) raise from the Cut-off with 3-3 to 400 chips. The Small Blind, who has more chips than you, pushes all in. This will cost you 2100 to pay in a 3950 pot. You must:
Definitely fold it... Your 33 is only good if you get another 3 on the board.
Chances are the person has a high pair which would have you behind already. If they don't have a pair and just have two high cards, its still a 50/50 to win it. So with no other information, your chances are 50% at BEST to win this hand with a 33, most likely its well below that if they have any other pair which would most likely be higher than your 33.