<human>: Can you condense the text into a few key points?

Hey all

My girlfriend (we'll call her J) has a close friend (we'll call her C) of hers who I am also friends with. 

Her friend (C) expressed interest in me, and asked my girlfriend if she would be cool with a threesome.

(J) asked me if I wanted to and I said no. I'm not really into the idea of having sex with people outside the relationship. (C) was sad but understood.

They hung out alone later that night and kissed each other after getting drunk.

Fast forward two weeks to today and I could tell (J) was hiding something because she was acting incredibly defensive. After probing her for some time she 'confessed' that (C) had asked to kiss her while they were cuddling but  she said "no."

I said that I didn't believe that was why she was acting really defensive and after a few more minutes of probing and laughing at how ridiculous it was for her to pretend that was a big deal she 'confessed' that they had kissed each other on the cheek goodnight.

I said that wasn't bad and I still didn't buy it. And finally she (probably) spilled the beans and said that they had kissed. 

She was very apologetic. This isn't the first girl she has kissed while we've been together (the other was her best friend).

She then said that she really wanted to have a threesome with me because "it would make you happy" and I feel like she has ulterior motives. I told her it made me uncomfortable but she kept pushing it so I agreed and now I feel like I was used as a means to an end. Originally I thought (C) wanted me but now I think she might want (J)?

Granted, (C) is going through a really rough time and really could use affection.
<bot>: Girlfriend kissed old friend of hers, they want a threesome. Both are attractive nothing wrong there. Feels like I'm being used.