How do I gain some control over these kids?

I have 3 boys ages 8, 6, and 5. The 8 and 6 year old both have ADHD. They fight all the time. They argue, hit, kick, push, and punch each other. They have left bumps, bruises and cuts. Our house is small and 2 of them share a room so it is hard to separate them. But when I do, they want to play together anyway. They are constantly tattling on each other. I hate the tattling because it is for everything from "Mom.... He spelled a word wrong." to "Mom.... He punched me in the stomach." and I never know who is telling the truth. I can't have my eyes glued to all of them constantly. We are a recently blended family, so I really don't want to make any of them feel like I'm choosing to believe one of them over another. But usually if one of them hit, it's because the other one hit first. I can't ignore the tattling because even when I ignore it about the small things or tell them not to tattle, they end up in a physical fight over whatever it was. I want them to learn to settle their disputes on their own sometimes, but I cannot let them physically harm each other. Oh, and when I do try to punish them with time out, they scream at me and tell me that they didn't do anything wrong. After a screaming fit they will finally take their time out, but the behavior starts right back up again within the next hour. I need advice. Please don't tell me to or hit. I have actually had a warning from CPS that I better never swat my child on the behind again.