Hareball from predfish has a Geryi tank with 6 or 7 fish, and it seems to work pretty well...nitrofish said:a tank of geryi's would look nice, but I belive it was clay that tried that and it didn't work out. I hope your situation is different.
I'd seperate the new one for a couple of days first, to let them get to know each other a little. But on the other hand, you'll never know what could happen once all three are together: I mean, maybe even seperating them for months cannot avoid agressive behaviour or heavy fighting. I guess you'll need a good dose of luck as well for it to work out (remembering they are serra's!)