HELP! Introducing a stray to other cats!

Hi, my family is trying to introduce a new stray into the house (we've only ever adopted strays). The new cat, Olive, is a young female cat (still a kitten, clearly) and we have two other cats, Manny (a senior male cat) and Sni (a male adult).
   
We're trying to introduce Olive in order to adopt her, and trying set up a safe room for her to be kept in for the coming week, so she can get comfortable with the smell of the other cats, who hang out in Olive's safe room. When she knows that Sni is on the other side of the door, she starts growling, but Sni, a laid back cat, just seems interested. Is this normal? Should I expect this same reaction to Manny? What do I do if Olive attacks Sni and Manny?

Hi, We have adopted many cats in my lifetime so hope I can help you too! Take it SLOW. Olivia may be intimidated being a kitten facing 2 adult Males.
   
Let them smell her thru a closed bedroom door for awhile, then let her out with supervision at first. Always be there to keep an eye on things in the beginning. Mayve 10 minutes then back in her room. Each day increase the time. It's normal for hissing and growling to happen. As long as no one is getting hurt it will all work out. How old is Olivia? It might be good to let her out by herself with the 2 males in a closed room so she can get a feel for your home too, and know her way around.
   
Other suggestions is have a cat treehouse in the main room so she can escape and go up high if she feels the need. It's always good to have cat trees for them anyway, they like the height!
   
Take your time and praise them all for getting to know each other.
   
I wouldn't leave them all together alone until you are sure the males aren't bothering her or getting aggressive, but cats adjust to each other well, so you have a good chance of a 3 cat happy family!
   
good for you! We have 6 in our home we either rescued or adopted from shelters too. There are SO many in need of good homes, so you and I are doing a GOOD thing!
   
Enjoy them and give kisses often! :-)
   

Oh, I forgot to ask-make sure your males are neutered? Hope they are by now.. And get Olivia spayed when she is old enough, 6 mos is good, even though many are doing pediatric spaying.. some vets say wait so their organs can mature properly and that for females is around 6 mos..