Dindin,

How long has your new kitty had free range of the house? If it has only been a couple of days, give it more time..cats can take weeks to months to sort things out between themselves. What's probably going on is that they are trying to establish a new hierarchy..who's gonna be top cat and that can take awhile to do. Even when it is sorted out, it is very likely that they won't all be the best of friends.

For now I'd suggest letting them "discuss" the matter amongst themselves. As long as no blood is drawn and its just hissing and spitting , they should be just fine.

Keep us updated as to how things go.