Basically it's whoever rules the roost. Willow is the lowest ranking, Molly is next up, and Buffy is the queen of them all. Basically the only way I know this is that both Buffy and Molly will boss Willow around, and Willow won't fight back (both of them can push Willow aside and help themselves to Willow's food, for example). Buffy bosses Molly around, helping herself to food while Molly just sits and waits. NOBODY bosses Buffy, if they dare try to take Buffy's food, they'll get a smack in the face instead. Willow cannot boss anyone around, since she's the lowest ranker, and she and Molly cannot boss Buffy.
Willow - cannot boss anyone around, is very timid and easy-going; if someone abuses her, she takes it all in stride
Molly - lords over Willow whenever she's in the mood, but doesn't dare tell Buffy what to do. Molly steals Willow's food and returns slaps if Willow is getting in a pissy mood. Molly does NOT dare return slaps if it's Buffy who's in a pissy mood, and Molly does NOT dare to steal food (she'll sit and hover, though)
Buffy - can do whatever she wants with the other two girls without getting any complaints. If anyone does complain, though, Buffy tells the cat where to shove it, lol
It's hard being Alpha, though. I can't remember the last time Buffy snuggled with one of the other cats. Once in a while Buffy will hop up to a snoozing Willow, wanting some lovin' (Willow raised Buffy), so Buffy will get a comb-over, and may eventually lie down and snuggle (but sometimes they get into a battle of wills, turning the grooming session into a fight). So basically Buffy's left out in the cold, usually snuggling with her own self. Molly's more of a loner, she spent a year being hated and abused by Buffy, so she doesn't trust Willow all that much either. Sometimes Molly and Willow will lie close to one another, but most often they don't. *sigh* Willow and Buffy used to be the best of friends, but I now think I killed that harmony when Molly came to live with us.