How do you stay under the board when you tre flip?

Ok. I have been practicing tre flips for about three and a half weeks and I have only landed six of them and they were all landed on the nose. The too main problems I'm having are under rotation and landing with my back only, but landing with my back foot only is my biggest problem with tre flips. The board will sometimes only spin 180 or 270, but most of the time I'll get the full the 360 rotation, but even if I do get the only 180, 270, or 360, I'll always land with my back foot all the time. The board will always be behind, way out to the side of me, or right next to my front foot when ever I land with my back foot. I can never get the board to stay under me. Any tips on how I can keep the board directly under me, get the full 360 rotation and land with both feet whenever I tre flip? Thanks.