With 3 kids, we always have some combo of 2 vs 1. The two ganging up changes based on age, stage of life and what each kid has going on. Sibling strife and fighting is the constant. We've had biting, punching, hitting and everything in between. I don't stress about it too much (other than they dance on my last nerve regularly when it's going on) because I also see them playing and working together. I grew up with 3 kids and we fought all the time. We all get along well in adulthood. My dad still talks about breaking up fights between my sister and me almost every morning while we were primping in the bathroom for school. She and I are close now.
I'm guessing your kids have a strong bond on a fundamental level.
Mom to Two Wild and Crazy Boys and One Sweet Baby Girl