I just adopted a new kitten and I already had 2 cats at home. The first thing you should do is take it to the vet to see if it is healthy- you don't want your 2 kitties to catch anything.
Then if you decide to keep it, keep him/her separated from your other kitties for awhile. Introducing them slowly really helps out in the long run! When I brought Rudy home, I kept him in the bathroom for a few days. It could take longer, it depends on the cats.

Good luck and I hope you keep it- sounds like the little kitty needs a home! The fact that it was waiting for you when you came home shows it must have bonded with you.
Keep us up to date on what happens