When we originally sat down and talked about this, yes it appeared as thought it was all or nothing. Some rules were put in place in the beginning about sex when one of the two of them wasn't there. Since then, trust has been built up, and those rules have been able to become a bit more lax. But, I do believe, that she would not be ok with me being his girlfriend and not hers. Of course, I haven't really talked to her about it as I am still trying to figure out what I'm feeling. She has made comments about how it seems like he really has two girlfriends rather than it being an equal triad, and I agree that this is how it appears to be shifting. But then, she'll talk to him when I'm not around about how she is afraid that his and my bond is deepening much faster than hers and mine. Which is true.
I agree that communication is key and that I should definitely be honest with her. I guess I'm just afraid of hurting her.