Dog keeps trying to hump my older dog

I have 3 female dogs, all three since they were puppies. This morning after I let the youngest and the oldest inside after doing their business, the middle one began humping the oldest one, which the oldest did not like and a minor scuffle broke out with the middle biting the eldest’s ear drawing blood. Both dogs are separated and will be for atleast the rest of the day. The middle dog has done this several times and I or my mother have usually been there to break things up before blood gets spilled, but I’m worried that I’m going to come home one day and find one of my dogs dead on the kitchen floor. How do I stop this? The middle child is definitely the boss of the three, second only to my mother.