help with getting along

I am cross posting this - so any suggestions would be appreciated.

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i have 3 cats twix,spike and millie cat, there is a cat that lives down the road his name is jasper, he has never been very well cared for by his owners, i have always fed him and left the shed open for him, on christmas day he came in my kitchen, (hes never wanted to before i have 2 dogs as well he doesnt like annie and nellie) hes not over fussed about humans thats not surprising cause his humans have not been nice to him. well to cut a long story short spike does not like him very much and he does not like spike, they hiss at each other in the kitchen, twix and millie dont seem that bothered, they seem to have stopped swiping at each other most of the time it does seem to be just hissing now, jasper seems to be a very angry cat who does not trust (that i can understand) how can i help him to trust me and help spike and jasper at least tolerate each other, i dont want to have to rehome him he is a beuitfull boy, sorry this is so long.
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First, she needs to have the new cat checked by a vet to make sure he doesn't have anything contageous that he could give her current cats.

Second, if the cats are just hissing and nothing else, that's a pretty good sign.

Third, if she is planning to adopt him she definately needs to follow the correct procedure on how to introduce a new cat to old cats. You might send her the information in this thread.