How much fighting is normal?

I have a bonded pair of brothers and a male I rescued a little over a year ago. I worked on bonding them for months and they are now housed together. The vast majority of the time they all do well together/sleep with each other. But there are some occasions at night or during playtime they will chase each other and have some fur slips. The fur slips are never by the neck, but are on their rump, and blood has never been drawn.
Is there some level of fighting that is to be anticipated like...

How much fighting is normal?