The line that Nyc quoted stuck out to me too. That's exactly how it has to be, due to human nature. Your third deserves to date individual people, not an established relationship. If a triad develops naturally, awesome. But having it as the goal right at the start puts a lot of pressure on everyone. Triads seem to develop serendipitously... after all, how common is it going to be that you find love with one person, let alone two, let alone two members of the same couple, let alone with both of them reciprocating?
And even if you do get that, there's often this pressure for everything to be "equal". But feelings and relationships don't work that way, they want to grow at their own pace, in their own way, on their own time. I can see why Angel was feeling uncomfortable if she was feeling the weight of that expectation, and *especially* if her new bf was smiling at her ("googly eyes"?) and she was picking up on the fact that her choices were to ignore him or contribute to the storm brewing between the two of you. How deeply awkward!
It's ok to need extra info and reassurance from S right now, but he would be violating Angel's trust of he told you every word she said... I know that my gf Gia shares some things from our conversations with her husband Eric, but if I felt I had no privacy with her ever, how could we develop intimacy?
There are some GREAT resources at this site that pertain directly to your situation. Check out especially the essay on dating a couple and the essays on dealing with jealousy. www.morethantwo.com
This can all be ok, but you have to let go of what you expected and embrace what's actually there instead. If that means that you and Angel are friends right now instead of gf's, or that you are gf's but you need to date without S around, that's ok. Just be real, and let him be real, and let her be real. Uncertainty is scary but it's way more authentic than a scripted scenario.