Moving to L3 is a big change in stunting. It seems like most people these days focus mostly on flexibility, but that's only half the battle. Without knowing any specifics it is kind of hard to offer many recommendations other than what is always repeated here, but I'll try! If you have any specific questions it is a lot easier to give tips. Good luck to your CP!
Good posture helps to keep a top in the air. Pull up through the ribs, elongate the spine, drop the shoulders, and chin up. Look at ballet posture to get a visual. Another thing that I find really important, but often overlooked, is glute strength. Strong core helps a ton, but hip and glute strength makes a huge difference IMO. I'm not sure how to embed videos from the app, so if this doesn't work search "fire hydrant exercise" on YouTube. Another variation on that exercise is to start on hands and knees and extend one leg backwards, like in an arabesque, then pull it back in to a bent position. And always do all stretches/exercises on both legs. I really like the app Sworkit because you can customize workouts to include strength in certain body areas or stretching and it makes it a bit more fun.
