Third operation on a wrist ganglion cyst to be done soon. Do I need to wear a support forever after this? I have cerebral palsy in same side as well.

Not for a ganglion. However if you have increased flexion of your wrist due to CP and there is improvement in your grasp and release with a wrist splint and there are no alternatives such as Botox or surgery and the splint does not interfere with your mechanics of grasp and release by holdong your wrist too tight or too exnteded then you may benefit form a splint. A splint is not typically after a ganglion for long.
Discuss with surgeon. This is one of those problems where a visit to your doctor is necessary to figure out what's going on. Only after a thorough evaluation, including examination and possibly labs and other tests, can your doctor correctly diagnose you and treat you effectively.