How do i cope with my 2 sons fighting all the time?

I have 4 year old and a 2 year old and i have just given birth to a 3rd boy 7 weeks ago. My eldest two fight constantly. They swear at each other, kick, throw things, spit and have recently taken to poking each others eyes out too! No matter how I handle the situation, they don't listen to me. I have tried talking, shouting, punishing, sending to room, but nothing works. I get them to apologise to each other and they do but then they are at it again! I live in a tiny 1 bedroom flat and i don't know if that has anything to do with it. My husband and I try and take them out to the park, grandma's etc but it doesn't change anything. The only thing that works is having one of them go and stay with his grandma for a few days. I cant keep doing this forever. I am at my wits end, I cry all the time and feel that I cant cope, I just feel like i ought to cope, (there are women out there with 6 or 7 kids for God's sake) but i cant and that just makes me feel worse. I do everything for the I have tried everything to distract them, games, PC, Art playing in bath, but all they want to do is fight and bicker over everything. This is affecting my care for my newborn because I cant sit for 10 mins to feed for having to keep jumping and separating those 2!!! How do I deal with this?