HI Jerry, my mum has the same predicament, she has two 12 year old male cat's very spoilt and over weight, she took in a small female tortie last October which she had been feeding for a while, the owners just didn't care or wanted her, she has a great nature too considering the awful short life she had.
Unlike you this cat lives in my mum's "bedroom" and is very happy and contented to be there, her other two cats know she is there but have they have yet to meet eye to eye so to speak, I keep saying to mum let them sort it out who's boss and things should settle down, up until now it hasn't happened, I keep saying to mum that it's not healthy for her or this cat to be up stairs all the time. X X X /// ^..^ \\\