Problem with my pitbull dogs?

ok I have three pitbulls one(male)is almost a year and the other two(one male and the other female) they are around 5 maybe 6 months now. Ok the one year old is seriously like the sweetest dog ever his never been rude to anyone not even strangers and he was the first one to enter our family the other two dogs came in 2, 3 months after he has been with us and he welcomed them in no problem they were six weeks old and yes I know that young to take them away from there mom but there wasn't really an opition. Anyways the male pitbull that's 6 months is snappy doesn't like to really be cuddly and the two male dogs have have been figthing alot lately why? It's always the small one who starts it too like how can I try him not to do it or be nicer? I don't think it's playing either cause usually the one year old wins by either making him cry and once made him bleed from his mouth but it's not his fault because it's always the one year old who starts it?