I don't advocate asking him to "back off" (which is not a clearly defined request). But you might ask for what you need to not feel so uncomfortable.
I suggest asking for partner care from both of them. Let them both know this is making you uncomfortable, talk to them and work at finding the reason WHY it makes you uncomfortable. Asking for less PDA in front of you for a short (week or two) is understandable while you work through this issue, but more than that could cause unhealthy strain between the three of you, and create resentment.
Asking and negotiating for what you need is ok, so long as you acknowledge to them that you see what you are asking for is going to be hard for them, and you show them that you are working on understanding how you feel. I am makings a guess that this is coming up because of a dose of NRE between A and B. If so then this
article might help you.
I wish you luck in working this our for yourself, and for your relationships.