Yes indeed: the triple backflip is still in the beginner category, because it doesn’t take much to do this trick.
Simply gain more altitude with your quadrocopter and hold the direction even longer. Make sure you are using a roll/yaw/pitch rate of at least 0.3 in MultiWii Conf.
watch on youtube: http://www.youtube.com/watch?v=SKYdVZTXmDM