Emergencies

ok, this is not really an emergency, but thought I'd ask the question here to see if anyone knows.

My neighbors moved and left their cat. I took it in as it followed us and needs a home. I already have 2 cats and so they are being kept in separate parts of the house from the new cat because they do not like each other at this point in time. They smell and hiss through the door. Anyone have suggestions to help develop their lacking social skills?

I understand this is natural and normal, just not sure if I should keep them apart with the door until they develop better relationships on their own or if there are ways to help this process along the way. Having separate litter/food, etc is ok, just wish they would not talk so nasty to each other.

The 2 existing cats in the household are 1 grumpy old fixed male and a grumpy old fixed female (both previous strays probably 8 years old) and the new cat is a female, young looking and obviously had a litter of kittens previously, who were placed in homes already (I know as another neighbor has one of the kittens).

Any ideas or suggestions?

they may and they may never accept the new cat seeing that the cats are older , its going to take time there is no easy way of getting them to accept this newcommer,, least not that i know of- with dogs it can be as long as a year or more befor the orginal dogs accept a new comer,, sorry wish i could help more , take care