Oh my goodness gracious hi! We are wresting with very similar questions right now. I recently met someone who is in a relationship. He's great, and he let me know the situation from the outset. The three of us get along great. We make a lovely little triad, I think. In fighting some of these So are we in a relationship now, too? demons, I find it helpful to (and get ready, cause it's so cliche) just talk.
Let me emphasize that. TALK TALK TALK. Whenever possible. Feelings of uncertainty and insecurity come along at the outset of any new relationship. But for some reason it seems like when stepping onto non-traditional grounds, communication is key.
That said, every person loves in their own unique way. My metamore tried to warn me that my gentleman (see, I'm not even comfortable saying boyfriend) isn't very outwardly affectionate. But we just had our first date (as in the three of us) and he kissed her, kissed me. Held her, held me. It was very loving and comfortable. She also mentioned this, which I've found the most comfort in so far: he wouldn't be risking his love and relationship with her for just anyone. I'm special.
I hope that was some help, and that we both weather these brilliant and exciting storms!