All you need is to warm it up for about 3 weeks. About 9 times out of 10, if you keep it warm, diacetyl will be gone in about 3 weeks, maybe 4 weeks at most.
Oh wow. After 3 months refrigerated, your yeast will be very very tired and should not be expected to perform very well even if you warm it up. You would be better off making a fresh yeast starter and adding it when it reaches high krausen, then keep the beer warm for a couple of weeks. Then taste it and hope for the best.It has been in the keg for 3 months now, and it has been stored refrigerated for that time. Do you still think the yeast will clean up if I put in in a warm place?
I recommend you add yeast as well as sugar, in the form of an actively fermenting yeast starter.That's why I thought about adding sugar.