Slot machine

hi,
Im tring to create a slot machine game…I have a mask.and I have 3 layers each of which has a motion tween of 120 frames.they animate my 3 slots…the problem is how and where I can assign a random number so that the motion tween should stop at any image.?
on(press)
{
slot1_mc.gotoAndPlay(2);
slot2_mc.gotoAndPlay(2);
slot3_mc.gotoAndPlay(2);
}
thanks