Well, I haven't had a chance to introduce myself yet... but regardless I thought I'd jump right in and open my big fat mouth!
My husband and I are in a long term, committed marriage. However it developed (consentually and with much discussion) into an open marriage and we've discussed the possibility of having a poly marriage involving another, most likely a woman.
As far as seeing each other being affectionate with others, that is something that (in my younger days) I would've considered unthinkable and would've made me insanely jealous. So, I've spent a lot of time thinking about why I no longer think/feel that way.
#1. Trust: There is a level of trust between the two of us that I've never had with anyone else. He's had many opportunities and reasons to leave in the nearly 8 years we've been married. If he wanted to be with someone else, he would've long before now.
#2. Love: I love him truly and honestly and I LOVE seeing him happy, love seeing him turned on. It eventually occurred to me that anything that turns him on, turns ME on... but I don't have to BE the one turning him on. I just enjoy the thought that he's enjoying himself. I don't even have to be there for it to make me happy.
#3. Respect: I respect his desires as an individual. I respect the feelings of the person he's with, when he's with another. I do not take for granted that I don't have to earn his love & trust, that they should come automatically just because we're married.
I see trust in the words you've written in your past posts, that she is being honest with you about how she feels about you and her other C. You've been married a long time, she hasn't left you for anyone "better" and doesn't appear to be seeking someone "better." You trust her enough to allow and even foster her relationship with C. So, the issue is maybe that you don't trust yourself? That you don't think you're good enough for her?
I see clearly that you love her. The key for me in learning to be ok with seeing PDA between my Love and another is focusing on his happiness, his pleasure, his desire... learning to take my happiness from his, my pleasure from his, my desire from his... you get the idea.
You respect the relationship between the two of you, between LR & C, respect your family structure as well.
The foundation is there... just build it 1 brick at a time. Be willing to take small steps, and to have a signal that will let her & C know if you aren't ok with something so that they can respect your feelings as you work on respecting theirs.
Good luck! Sorry for being so long winded... I type fast!!