pretty much everything is said, i struggled with 3s a long time (i could land 9s before 3s) and always landed backseat, it comes from popping off not straight up, and from spotting the landing too early, which means u look down, this drags u into backseat. so focus on a good take off, try to rotate ur shoulders with ur head and not only the head towards the side! try to spot url landingpoint (looking down) pretty late.
what i think is the the problem that u dont come around: YOUR ARMS! i experienced this with a lot of kids. u actually jump off correct, but instead of having ur arms in the direction of the spin all the time, u start to move them against the spin in the middle of the 360, and this stops ur 360 and its hard to get around! so try to film urself and focus on ur arms...so u see what u do wrong! a grab will help, but prolly too hard in the beginning.
i hope i could help u, and sry my english is not the best!