Re: Adopting a kitten and already have 2 older kitties...
I went through this exact same experience when I rescued my cat Leo from outside. I had two older cats at the time, Mac (who was about 13) and Graylord (who was 3). Leo was around 6 months old when he joined our family, and at first I kept all three totally separate from each other. Now I don't know if its best to have the same sex or not, but for my cats I think it worked out better with having three males, then two males and a female. What I did was, on the first day the kitten comes home, bring one of your cats in to spend time with it. Now the first time should only be a minute so they just know there is another cat in the house, and can recognize their scent later on. The next time make it a few minutes, and supervise the encounter to make sure there are no attacks.
Once the two cats get on a friendly basis, bring the third one in and do the exact same thing you did with the first. Slowly they will start to become familiar with the kitten, and slowly but surely become friendly, or at least tolerant of them. I'm not saying this is going to be the case 99% of the time, but it worked for me. :/
Rest in Peace Mac, I'll always love you!
"If man could be crossed with the cat, it would improve the man, but it would deteriorate the cat." – Mark Twain