Ok, here is MY take on it, and you can do what you want to with my opinion.
PERSONALLY, a couple needs to spend time TOGETHER in a monogamous relationship for quite some time BEFORE delving into a poly relationship. This helps to build the trust necissary in a poly relationship. It also helps to build the FRIENDSHIP that the couple needs to be successful. My wife and I had been married for about 7 or 8 years IIRC before we brough anyone else to our household. And even then, it was a good friend that we BOTH new very well.