Thank you for wanting to get a play-mate for Mikey. I'm going to assume that Mikey is male here, but at 3 years old it then I would generally say that a female would be better for him. Then you won't have to deal with alpha-male issues and such, even though your cat has been living with other cats, these are cats that he "knows" and is familiar with....new ones may or may not present issues, and for that reason I would suggest a female. Mikey may still have issues with the female, but "generally" they will bond faster and likely closer if they are opposite sex and you find one similar in disposition.
Notice I said "generally" because all cats are different and you have to gauge the attitude of your kitty. You know your kitty better than we do, so you will need to consider...how does he handle "new" situations and new humans? Is he shy? Is he boisterous? Have you ever introduced him to another unknown cat before? Is he mild mannered or is he territorial?
I applaud you for adopting a buddy for him. It does quite normally take some time to assimilate cats together when they are unknown to each other - and that means male or female, although my experience "generally" shows male-female relationships the most successful in terms of how quickly they will bond (since the alpha-male issue isn't there).
I would be more than happy to post any tips or answer questions if I can help with any other specific question.