Before you try this trick, try to get better at kickflips, varials, and 360 shuv-its. since the three flip is just a 360 shuv-it and a kickflip put together.
Step2
When you first start, keep your speed at a comfortable one. then make sure your front foot is at an angle pointing your toes towards your nose. Your back foot should be in that little concave on your tail, right where your board begins to curve upward.
Step3
after that you scoop your back foot and send your front foot foward. if you do this right you should have the board stay relatively close to you and do the whole rotation.
Step4
when you do the flip you will sort of jump towards your back right foot. keep trying this and you should get it done. then try doing it off some ledges or onto a manual pad or something...
http://youtube.com/watch?v=7vowfuhjXCg he knows his stuff
Tips & Warnings
get your kickflips, varials, and 360 shuv-its down. otherwise you'll look like your just flipping and tossing a board around. which doesnt impress anyone.
skateboarding hurts if your stupid doing it, so don't rush anything. broken bones, torn skin, and big bruises are all a given in skateboarding.