OVER SPLITS! They are your best friend when it comes to your bow. For your needle sit in splits and do your scorpion. Once you can hold that for at least 30 seconds start to straighten your leg. Eventually it will increase your back flexibility. Once your leg is completely straight hold it for at least a minute to 2 minutes. DON'T FORGET TO BREATH! Once you can hold your needle for 2 minutes you can then put a yoga block or something under your back knee to lift your back leg and eventually get a hyperextended needle. Hope i helped!
If you can already do the splits, then try making it even harder. put a pillow or something under each foot while in the splits. That will stretch even past the regular splits. It really depends where you want to be more flexible, but it also helps to do the cobra stretch. Though be careful not to hurt your back!
You are already quite flexible, even some people on the drill team at my school can't do splits both ways! That's pretty amazing to be able to do the splits 3 ways! :D
Try Gabi Butler's stretching videos on youtube, they are great :)
I got my flat scorpion in a week after stretching
Try yoga! It's fun and would really increase flexibility.