I'll get my small rant out of the way:
You want a girl to be join you as a couple. You and everyone else. I'm a potential unicorn, that is I'm willing to date a couple. I see these ads all the time. And NOT ONCE have I ever seen the couple showing me what they have to offer me. It's all about them.
Dating a couple is risky. I might fall in love with the husband, and get kicked out by the wife, if I refuse to be intimate with her. I might develop attachments-- and find it difficult to find a "legitimate" partner who is accepting of my poly relationship. I might have issues telling friends and family. Unicorns get burned, frequently, and as a result, can shy away from future attempts.
It's a big thing you're asking for; what are you willing to give in return?