sirjam, I noticed this too. What I've found is that when your guy with the ball (a) has not used his dribble and (b) has no one on D within arms length or 5-sec call length from him, you can perform the triple threat moves. As soon as a defender moves into your personal space, the dribble animation takes over, even if you do not move the left thumbstick at all.
IMO, this is a glitch by EA in the sense that the triple threat move is useless if you can only perform it while there is no one guarding you closely. Thankfully there's so much more EA offers that it's not a huge thing, though I hope they fix it for 07.