Hit a bigger jump.
But, yeah. Have your feet about shoulder-width apart, pop off of the jump, then turn your head and shoulders in the direction you want to spin. Try not to lean as you go off- you'll get thrown off balance. as you come around to land, open up your stance to slow your spin down. Then land and ride away :).
do you have your 180's dialed in? My mistake in learning 3's was i skipped right passed doing 180's. Getting these on lock is key to getting used to the feel of your body spinning in the air. just do em on the flats... it helps so much.
Aproach the jump as if you're going to throw a 180, visualize the full spin(like mentioned above), set the spin a little harder, but smooth. There's nothing worse than a last minute nervous jerk to set a spin.
When i do them i approach the jump straight on with my feet about shoulder width apart, and when i pop i spin my torso and my head at the same time, and my feet follow. DON'T stop looking over your shoulder untill youve come around and its time to spot your landing. A lot of people tend to stop looking over their shoulder at about 180-200 as their feet come around, and can't complete the rotation; thats probably why you are stopping early.
Just keep trying and make sure you keep your head looking over your shoulder and youll have them in no time