My sister has a 3cm ovarian dermoid cyst and may need to remove it. Can she have heavy exercise in gymnastics until surgery? She's 14.

Don't see why not. This small cyst does not place her at significant risk during her normal daily activities including exercise. While she will face restrictions for a while after surgery, I doubt anyone would consider restricting her before it. The question would be best posed to those will all the information about her case. .