How can i stop my kitten attacking my two younger kittens?

I have a 11 week old kitten who I got at 8 weeks and brought him home and introduced him to my 4 year old cat. All was well until today I rescued two 6 week old kittens and he keeps attacking them. Are they playing and i'm being too protective or are they actually fighting?(all kittens are boys) Also, will they get better? it is the kitten who is attacking/playing with the younger ones