What Dagferi said...
Seriously, this isn't going to go well, unless you can be out in the open, with full support from everyone, and nobody treats you as a dirty little secret. But it doesn't seem like the foundation is there, right now.
Frankly, I think you're 18 and have SO MUCH LEFT TO DO in life. The time to be in a poly relationship like you're describing is when you're bringing a rich, passionate set of experiences to the mix. Right now, and no insult is intended, you're bringing youth and that's great, that's wonderful, but it doesn't last. If you want to explore and have something last, go live your life and establish a foundation from which to build something wonderful.
Sadly, this doesn't sound wonderful.