I have 3 devices connected using same apple id, i want to isolate my game centre id, without loosing my old games

IF you use a different Apple ID with Game Center, you lose the progress in those games.

The only way I see is keep that AppleID for Game Center, and get a new AppleID for everything else. of course, all your previous purchases, and your current iCloud and iMessage accounts will remain forever tied to the old AppleID so you cannot migrate any of that to the new AppleID. So no matter how you slice it, you have to give up something to separate thInge.