Focus more on developing relationships/friendships with people individually, and you will probably have a better chance at meeting someone who is open to being poly with both of you. Try and meet people who do things you are interested in, out in your community. It sounds like you have been focused on the sex (ie., calling it a threesome), and so it seems you are attracting people who have rather shallow goals and aren't being upfront at first about what they want.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein