I started a thing called the "band of brothers" for my boys this summer. I sat them down and discussed with them what it really means to be brothers. I gave each of them a rubber arm band that identified them as part of a team. Every single time an argument starts, I gather all 3 together to sort it out and remind them that they are a "band of brothers."
I admit I was getting really sick of my own speech, but slowly things have really started to turn around. Over the last few weeks I've noticed a willingness to work things out for themselves and a tolerance for quirky behavior. I'm hopeful that it will continue, it's definitely an improvement over the past!