Welcome to our forum.
"Mono with a poly (does that make us monopoly?)"
LOL, well it doesn't make you "a monopoly," but it does make you a mono/poly dyad.
Mono/poly pairings are challenging, but I know it can be done; I know people who have done it. Communication is a crucial key in what will make the difference.
Also, whatever relationships A has, hopefully she'll keep an eye on that NRE (New Relationship Energy) factor, and put extra effort into her relationship with you (so that you don't get that "left behind" feeling).
Anywayz -- glad to meet you here.