Thank you all for the help. Things have turned out beautifully.
C is back with his ex, K. I still want to talk to him about what's up with D, but he's been stressed lately so I've been avoiding bringing it up.
K is the girl he's in love with. Though K is more or less mono, she supports his relationship with me and thinks we're good together. I know he cares about her more (mostly, I think, because he's known her longer), but she and I have had a couple of talks about jealousy and she's been making an effort to learn more about poly, and we've been building a friendship.
On C's birthday, K and I baked a cake for C, then came up with a devious plan to get him in the shower (a birthday tradition in my dormitory). We tied him up and with the help of a few others, wrestled all 6'8" of him into the shower. It was great fun. The three of us hung out, and he made an effort to include me physically - tickling, hugging, massaging, etc. I felt cared for, and it was great.
On Valentine's Day he took her out for dinner. Later that night he was hanging out in a friend's room, and I came and joined them. He called her in and I cuddled up with her, and she stroked my hair and told me how glad she was that we're friends. I had my legs across her lap and in C's lap, and he tickled me and she stroked my legs and back.
All in all, things are going well. I'm still not sure what's going on with D, but nothing feels threatening right now.
Me: 22-year-old female, cis and queer, have identified as poly for ~2 years, currently in my first committed poly relationship
A: Poly boyfriend since 9/17/13, currently sexually open and not seeing other romantic partners but open to such in the future