Should I be worried about the relationship?

My boyfriend and I have been dating for about a year, we both love each other very much. Lately he's been acting obviously strange around one of my girlfriends and I didn't bother to think about it until he told me that he's liked her for a while. He wanted to tell me this because he didn't want to hide it from me. He reassures he loves me and I have nothing to worry about. Although he has asked me if I was wiling to have her join us in a threesome, in which my answer is strictly no to that. It's weird for me now every time we all hang out together. I know he is with me, that he loves me, but should I be worried about the whole situation? Or am I just over analyzing it?