How long have A and C been together? You say it's recent. It's a new relationship.
You don't feel threatened or jealous because you've had other lovers for years and you and A both know either of you having a lover is not a threat to your relationship.
But, having C in your lives is a change. There is a new dynamic. Neither of you can yet know exactly what C brings to your poly family. Heck, my gf has her first real bf now, since she and I have been together. (We've been together 2 years and she started seeing M last November.) I am still getting used to what it means to share my gf with her bf, and even she and he are still in the exploratory period of what they mean to each other. What they are learning from each other. How they are growing as people.
So, perhaps what is going on for you is, you are just feeling unsettled. A new person is in your family boat, and you all need to rebalance in your seats.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
"Master," (mono, 34), miss pixi's Dom for 2 years