With poly relationships and coming out, it can be good to think about the nature of the relationship you have with an OSO. If it's a pretty stable feature in your life and doesn't plan on going anywhere, it can be good to just say, hey look, this is my life and this is how it is. But if the relationship is still forming, it can be pointless to go through a lot of drama and possibly pain over something that may or may not be around in a few months. It also depends on how your parents might react. If they are relatively sex positive, gay friendly and openminded, you'll have an easier time most likely. You know them best. You could always bring up poly in a neutral context to here what they might think about it... Testing the waters as it were. Coming out can be good and I know how hard it is to have a relationship in the closet. Since you're not financially dependent, you've got a lot more freedom.
keep us posted!