Gosh, I remember when I was in my late 20s - all my relationships were filled with drama and angst. I wonder if that's just par for the course at a certain age. I remember my astrologer telling me, "the 20s are all about fucking up" (and learning from our mistakes). Anyway...
I just think you all need to communicate more - a pow-wow with all three of you present to talk about how things are going emotionally for each of you, perhaps on a regular basis. If your relationship's in trouble, I would step carefully but examine it and really get clear on how everyone's feeling. That seems to be the thing that has messed you all up before -- lack of communication, or not enough communication.
You didn't mention how much socializing you do together, but I also think that maybe doing fun things as a group of three, and also just you and C, might bring some more of a sense of cohesion and stability to you all. Just my intuition telling me that.
All the best to you...
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein