can't say for every server, but on our server there are only two people with a chimera (I'm one of them...beams with pride) and if I check the mount sales it shows two left. I gather from that that the three limit is for each realm and not the server. I'm the only one in my realm with a chimera, the other person is in a different realm. Jumping realms might give someone a chance to buy a sold out mount. Only way to know for sure is try I guess.