We had three at one point, a Staff, a Yorkie and a Lancashire Healer. ( all bitches )
From experience one always had to be the boss ( the Staff ) and every now and again, there`d be a big scrap somewhere in the house, where the Staff would be putting the Yorkie in it`s place - who was like the little fella in the pub, who thought he was much bigger than he was !
The biggest problem I found, was the costs of putting them in kennels and finding the right kennels that would take the three of them.
Other than that, it was great, as all three of them had different personalities and used to rub along well with each other ( most of the time )
There was only the Staff that wanted to go out, so I`d take her for a walk, whilst the other two stayed happily in the house.