European Burmese kittens & cats in Fountain Hills, Arizona

I adopted Odysseus (Ody) from our family farm when he was 8 wks old (he turns 3yo in June). He's been a good companion for that time and is a good cat in general. Like most cats, he hides from guests for about a half hour before coming out to demand petting. He likes to be both indoors and outdoors. As a kitten, he was always outdoors or in the (large) equipment garage where there were many bugs and critters to hunt. Ody has been inside 95% of the time because I lived in an apartment or a townhome. He is litter trained and doesn't have accidents other than a hairball puke now and then. Unfortunately, Ody does not get along with other cats. My girlfriend moved in with her two older cats (13yo & 10yo) about a year ago and we have been unsuccessful at integrating the three of them. I like my cat and don't want to do this and have run out of things to try.