ok first of all DONT just put them in the same room and if you do and they start to fight DONT TRY AND PULL them apart. pits and boxers are MEAN fighters. they latch on and dont like to let go. they can hurt you really bad if they fight.
put the new pet in a cage or something of the same type and let them come up to the cage and sniff each other. or hold the new one and put the other two on leashes and have someone holding them to were if they try to fight they can move them to a different room
Why do people say that pit bulls "don't let go?"
Through selective breeding, pit bulls have developed enormous jaw strength, as well as the "hold and shake" bite style, designed to inflict the maximum damage possible on their victims. This bite trait delivered excellent results in the fighting pit. When the Colorado Supreme Court upheld the Denver pit bull ban in 2005, the high court set aside characteristics that pit bulls displayed when they attack that differ from other dog breeds. One of these characteristics was their lethal bite:
"[pit bulls] inflict more serious wounds than other breeds. They tend to attack the deep muscles, to hold on, to shake, and to cause ripping of tissues. Pit bull attacks were compared to shark attacks."11
Leading pit bull education websites, such as Pit Bull Rescue Central, encourage pit bull owners to be responsible and to always carry a "break stick" -- a device used to pry open a pit bull's jaws -- in case their dog "accidentally" gets into a fight. These same websites also warn that using a break stick on any other breed may cause serious injury to the person.12 This is true because no other dog breed possesses the pit bull's tenacity combined with a "hold and shake" bite style.
One of the most powerful examples of a pit bull "not letting go" occurred in an Ohio courtroom. During the Toldeo v. Tellings trial (Paul Tellings attempted to overturn Toledo and Ohio state pit bull laws and failed), Lucas County Dog Warden Tom Skeldon showed a videotape of a tranquilized pit bull hanging from a steel cable. The dog is unconscious and still does not release its grip. At the time of the taping, the animal was being housed at the county animal shelter.