Introducing another kitten?*help quick*

Well I know it can be hard to introduce cats to each other. but currently we have 2 kittens. (sassy recently passed away). My mom found this adorible kitten that looks only about a week older then my two.

Is it difficult to introduce another kitten to my two kittens. My mom really loves this kitten but she doesn't want to cause problems. What are the chances of them getting along with out to much trouble. It seems liek it would be easier to introduce a new cat when they are younger then when they are older.

How old a kitten?
I added a third four month old to two other four months olds, and other than about five minutes of them looking suspiciously at one another nothing really happened. I never even separated them at all.

(You know those aren't kitens in that photo, right?? )

lol yes I know those are 2 of my hams Trixie and Krista.

No my kittens are Ginny and Hermione:

They are about 11 weeks old. So the other kitten is about 12 weeks old. So it should be okay?

It's OK as long as they are all healthy.
Where did your Mom find the third kitten? Is it sneezy or anything?

My mom found the kitten at petsmart. The kitten is in great health it has its first shots, is dewormed, etc. My kittens are also in good health.