Our Basenji is about 65 years old, we also have 2 other dogs, a female black lab 6 years old and a male 1 year old Boerboel (Mastiff). They all get along really well with the Basenji being the alpha dog still. I have noticed over the last year that the Basenji will put his rear end in the other dogs faces and jump in the air and hit them with his butt. If this a dominance thing? I have never seen another dog do this, ever in my life. Please advise. We are also going to have a new Mastador pup around Thanksgiving, do you think 3 males will have dominance issues with each other?

First, yeah they do the flip and butt in face. I see it as invitation to play/chase mostly.

Kind of shaking my head here over the rest. You already have a breed known for dominance and protectiveness of property/family with the Boerboel. Basenjis (I assume you mean 6.5 not 65 years old :) ) are not known for tolerating same sex. Sometimes they do, but not a sure bet. Plus, your Boerboel is still a puppy. By age 2 or 3, that compliance with the Basenji being alpha can go down the drain at a drop of a hat. That happens, and he hasn't killed the basenji... you have to keep 2 packs running, ensuring they never have contact. Then you bring in a mixed breed... which is all a Mastador is. No way on earth to know how much it will take after the lab side or the Mastiff. AND it's another male. So now you have potentially 3 dog aggressive male dogs, with 2 of them large enough to inhale the basenji. I am not sure about the breeders who placed the Boerboel, certainly not the owners letting you bring in the Mastador.

I want my male dogs to have a chance of a peaceful, unstressful life. I think the chances in your home are already approaching really low numbers, and the Mastador is going to bring that closer to zero. The safety of that poor basenji is beyond precarious. I wouldn't want those 3 males in the most experienced of homes, one already fully ready to and experienced in running separate packs. I sincerely hope you reconsider the Mastador. And no, getting a female won't help. Until the Boerboel is fully mature and you have a handle on what your pack is, adding any dog is going to up the risk of issues.