^I wish I'd seen this video when I was learning them.
I cannot stress how helpful a trampoline is for learning flat 3s, learn the rotation inside out on a trampoline before trying it on snow and you'll save yourself a lot of pain!
I had the same problem as you when I was first learning them, I just threw wackflips which looked, and felt terrible. To overcome this I stood on my trampoline and jumped backwards landing loosely tucked on my back at an angle 90º to my takeoff. If you imagine the trampoline as the face of a clock I stood at 12 then jumped and landed on my back with my head at 9 and ass in the middle of the clock. Then using the bounce of the trampoline carryon and land facing the way you started at 6 o'clock. I then practised that motion throwing it a little harder every time and eventually I got the hang of the flat rotation. Then I just took it bigger and bigger on the trampoline and threw grabs in and more spins.
I've always thought that flat 3s are by far the coolest trick and when you land your first one the feeling of awesome you get is unparallel to anything I've ever experienced.
I truly hope this helps!