Hi Welcome...I am pretty new to the site too and am thankful for it, as it provides a great sounding board when the navigation of an "Its Complicated" relationship status becomes difficult.
I would be the equivalent to the wife on the other side in your situation. My hubby and I have been together forever....since I was 19. We have never cheated on each other and decided recently that maybe, just maybe, you were NOT suppose to F*** the same person for your entire life! So, we asked a friend of mine if she was interested...and um, 3 months later, apparently she was!
She, C, always claims that she is the 'interloper', the 'intruder' and claims that she feels left out of decisions. Sometimes claims that she feels like she is walking on egg shells so that she doesn't piss me off, ie, wanting alone time with the hubby.
Something that seems to come up again and again, is the fact that we are such close girlfriends (no kidding!!), and she knows me well. Maybe toooo well. She picks up little nuances in my behaviour and my actions, when my words were never spoken. Ironically she has a pretty good success rate of reading me. Note to self: must practice poker face!!
This week we have all agreed to have an actual "Communication Meeting" once per week in order to flush out some of the realities and clear the imagination cache! So I would highly recommend this for your new triad as well.
The highs are sooooo high....and the lows are soooo low. Be prepared and do what you can to avoid some of the pitfalls that we have gone through.