I think a lot of couples who are starting to explore polyamory think that a tried would be an ideal relationship because it seems less threatening than dating separately (like maybe you won't be jealous if your partner falls in love, if you are also in love with the same person). In reality, it doesn't work that way. Solid triad relationships do happen, but it's sort of a fluke-- there are far more people looking for them than living them.
It's a difficult thing to achieve, and if your wife is not bisexual, that's going to make it a little more of a challenge. Or do you envision your wife simply having a platonic friendship with your other girlfriend?
(Your intro post suggested that you are hoping to bring in another, younger girlfriend who can have a baby, so the three of you can co-parent together. That's a pretty tall order.)