I'm not currently in a full on poly relationship yet, but I have discussed it with my boyfriend. He actually brought up the idea of a threesome one day, and I jumped all over the idea. While we were talking about it I ended up bringing up that maybe perhaps we could "have another girlfriend". I was sure to inform him that while I'd be happy with a woman I can be happy with him too. He's the love of my life so if he were to say "a threesome is fine, extra girlfriend no" then I'd accept this and respect his choice. Luckily he was pretty understanding and he agreed that it would be an interesting idea. Though we did agree upon the fact that: no guys. While I would be fine with it but my boyfriend would not, so we cant have "extra sausage"
I dunno, I guess the most I can recommend for coming out to someone is be clear, be honest, and be aware of how you're phrasing something. Tell them what you want, ask them what THEY want, and hopefully your desires coincide. Most would be afraid that another person would be a replacement, but you have to assure them that its merely someone else to love. Remember: "not just mine but yours too" If they're okay with another person, let them know that you want them to like the other person too. So talk with them about what gender they like, what relationship (just sexual or just affection?) they like, or if they even WANT to be anything more than exclusive.
Though something of note If you want to be ONLY in a poly relationship you have to be careful not to start a monogamous one. Leading people on is bad, so if you don't want to be with just one person: don't be. Also make sure you aren't actually just seeking an open relationship. There's a difference between dating someone while you both sleep around with whoever, versus being committed to more than one person.
The post above me has a good point, but I won't waste space repeating what they said. So I just recommend you heed their advice.