man, backtail 3shuvs are definitely the hardest thing to do in this game, at least for me anyways. Someone explained to me how to do it a while back, and it definitely has helped me getting them, although it still takes me like 40 tries to do. Here's what he told me.
pop the ollie and then let go of the right stick.
when you rotate into the ledge, make sure you only turn using the left stick.
when you are about to get into the slide then push straight down with the right stick
hold the right stick down and rotate it up to about the 10-11 o'clock position (you can't rotate to fast, otherwise it'll make your guy pop a shuv the other way)
then once you get to that postion do the 360 shuv flick.
the trick is to hold the right stick down the whole time after you lock into the tail. hope that helps. It is easily the most satisfying thing to do in this game.
damn, I'm such a nerd.