My problem on three's just before about 2 weeks ago was that:
1. I would start my rotation on the jump (before popping) which would cause we to fall backwards and either I would land really far back-seat (raping my shins), or landing on my back.
2. The other thing I didn't do was looking over my shoulder the whole entire way, this helps increase the rotation and it would help me get the full rotation every time.
Now, after fixing some of those problems, throwing 3's off jumps that are around 25ish feet are a lot easier.
Remember, pop, and keep turning your head around your shoulder until you can spot your landing. Try them off smaller stuff and then proceed to progress.