Some slightly more nuanced advice -- talk to your wife first. Actually, scratch that, read the relevant essays at the site I linked above first. Don't assume you know what anyone else feels or wants, just communicate. And try to avoid getting a fixed idea in your head about how the relationship needs to look, who else your gf can or can't sleep with, how fast things should move, etc. Reading other people's stories here may help too... we've got lots of married couples and people dating one or both members of a married couple.
Oh, which leads me to another thing... *don't* think that you two have to love the gf equally or she has to love the two of you equally. It's ok if one relationship develops and another doesn't. Emotions usually work like that.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.