I am 17 and 5'6", i know its all genetic but what can i do to atleast get 2-3 inch, some say boys grow til 21 but i have not grown much since last year?

Growth centers: Your bones mature at a rate that is usually identical to your age, yet some mature faster & some slower. If your bones are more like a 14yo then you might grow another 2-3 inches, but if they are like a 17yo & you have slowed down over the last year you are likely not going to grow much more. An xray if your left wrist & hand could tell you if you will grow more. No diet or drug will help.