Flat 3s on tramp are pretty easy to learn! There's lots of videos that'll help. For me, there were only 2 parts to learning the progression of it.
First part is simple. I assume you can jump up and land on your back and pop back to your feet. So do that but jump 90 degrees on to your back and roll over another 90 so you're facing the same direction you started. This gives you an idea of what rotation your body will do. After doing this a while, I put an emphasis on dipping my right shoulder(spin/flip left) in order to spin the 90, rather than just turning 90. This is sort of how you'll set it so you actually go off axis.
After that you can start to actually throw the flip. Some people say to take it to your knees but it doesn't matter. You can throw it to your feet or knees the same way. So you want to jump up, dip your opposite shoulder(i.e. If you spin left, look over your left shoulder and dip your right shoulder) and reach for Japan/your foot. This gets you on the right axis. I like to think of setting more of a spin than a flip, but on a tramp they usually come out more flippy.
Some people do a backie with a Japan and that isn't actually a flat spin. Once you do it a few times you'll get the feel of it and it'll be easy!
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