Cat introductions - making a duo a trio?

I have recently come across a cat at the humane society that is putting me in a complicated situation. This cat is so sweet, so people friendly and she reminds me so much of a past cat. She looks identical to the two cats i have now that they could be triplet sisters. So here is the problem that concerns me. My cats are super friendly cats, they are sisters from the same litter, and are very pive and people friendly as well. Am i going to have problems with 3 female cats in the house? I hear that females dont always get along with other females, but that it also depends largely on the cats personality. I babysat a friends male cat once and they could have cared less about him (he kinda had a stick up his ). But this whole female to female thing worries me a bit? Has anyone introduced another female into a 2 female home before and did it work out? Or does it just depend on my cats personalities? Thanks! :)