I have to admit I am a little worried Nala won't be welcomed by Kiara and Sarabi... they're obviously very close and have been since they were born, I just hope introducting another girl in the trio won't result in jealousy...
It shouldnt be because they are so young and so much more accepting, but there are things you can do to help them.
You can put vanilla at the base of their tails and under their chin, so they will all smell the same, since cat scents are so important to them.
If you do a search introductions on here you will find a lot of useful information on making it easier for them.
Here are some things to also read over, you can scroll down and see what is most important to you now.