Kira practically took the initiative for the 3some, they were all drunk, but I don't know if Kira did it because she really wanted it, or rather because she knew the tention between the other two, or maybe she tried to prove something, I don't know. Anyway, I think Kira feel hurt by Nina's actions/behaviour/closeness to others, or whatever, and Nina knows she is hurting Kira, that is why there is so much flatter after a fight.
All this is what makes this apartment amongst the best: They are so natural, as if the cameras never existed, and we get to follow their intimate and emotional life, wee see their strugles, fights and dissagreements, but also passion, desire and strong love.
From what I observe, most my sympathies are with Kira, and I too admire her!