This sounds lovely.
They say that the most successful triads develop serendipitously, and usually out of friendship. The trick now will be to communicate, stay flexible, and deal with issues of jealousy and insecurity as they come up. You're off to a great start, and while there may be unexpected bumps ahead to overcome I don't see any red flags here.
This article may be of use to all three of you: http://www.morethantwo.com/coupledating.html
Best of luck!
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.