It's polyamory, not polysexuality! So yes, it counts. However right now you could be either mono or poly yourself, C is the one who is being poly. If you had sex elsewhere but still only loved C, you'd still be mono. Do you know which you are?
I mean, it seems to me you have a metamour relationship with B, not a romantic one, but I could be wrong!
Either way, it looks like you're a great vee and I'm really happy for you about it. It seems to be going right, and the absence of jealousy is always good, although jealousy doesn't mean you need to stop everything either, as long as you're willing to work through it.
I haven't been in your situation but it seems lovely, since everyone is happy ^_^.