im assuming you're talking about a hs fs 3...
1) make sure you go up first before you spin (get your height)
2) pull yourself to the handle. i used to do my hs fs 3s with my hand waaay out there - i could get the 2/3's or so and then i'd just land on my ass. if you don't pull the handle in there's no way you'll get the handle pass, which makes it really difficult to complete the trick.
like others have said, it's good to practice. either on the ground or on a trampoline.
when practicing on ground or tramp, don't forget to concentrate on pulling the handle in and advancing your body forward. i first practiced them on the ground AND on a trampoline, but didn't think about pulling myself towards the handle - and it didn't make much difference. then i went back to the trampoline and concentrated on pulling my body towards the handle, and the next time out i got it.
it's easy to do them on the ground w/o advancing, but that's what it's like behind the boat as it's always pulling you forward. if you concentrate on this aspect of the pass you'll find it a lot easier behind the boat.
1) make sure you get your height first - just like a 180, don't try spining before you leave the wake.
2) pull your body to the handle - this makes getting the handle, and thus the rest of the rotation, much easier.