I had the same problem after high school with the transition to a bigger court in college. The way I adjusted was I had to use my legs more. In high school I easily shot the 3ball with a flick of my wrist but against better competition in college, I had to adjust by using my lower body more when I shot. Basically just bend your knees and try to get your power from your legs into it when you shoot. I am bad at explaining it through writing but I hope that helps.
one is practice FTs, like another poster said, bend your knees down and go up THEN lift up your shooting pocket then smoothly release. Imagine it as a flow of energy gradually going from ground to up.
another one is try to maintain just the way you shoot but practice shooting 2-3 steps behind the 3point line, if you start shooting these shots, soon the 3 point shot is going to feel much much easier.
Well I'm fifteen as well and am pretty skinny and not very strong. But i can shoot the three pretty well. It's just more of a mental thing; once you see the three point line you think you're so much farther out. But if you just shoot normally, use your legs, you'll be surprised how close it really is.
Like was said eariler, start close to the hoop and shoot from there until you are consistent, than move back a foot or so. Also technique is important. Have a slight bend at the knees, and when you reach the apex of your jump is when you should shoot. If you're right handed, keep your left hand on the side of the ball to guide it, and vice-versa. When you shoot, you should have a nice flick of the wrist and the ball should roll off your fingertips. You should also be focused on where you are going to shoot at all times.