Welcome to the boards!
Just so ya know, you've likely got a long search ahead of you. The odds of finding a woman who is looking for a couple to join are exceedingly slim. Indeed, in poly circles the hot bi babe who will fall in love with both halves of a couple at once and live happily ever after with them is referred to as a "unicorn." Yup, a mythical creature that doesn't exist.
The bi, poly women who do exist report that they're generally not likely to get involved with both members of a couple at the same time and they doubt they'd grow as close to both equally. You're much more likely to each find another partner and perhaps have one of those grow into a triad.
That said, I know folks who had exactly such a triad, so it certainly is possible.
OKCupid allows non-monogamous folk to use its services. I know there are poly folk with profiles there (my wife and I have profiles). There are (or were) a couple of poly-specific personals sites, though I'm not certain if they still exist. Um, Polymatchmaker is one, I think.