Our group does three left, one mid and two right. That will work on both normal and hard difficulties. You can also keep the exact same formation when defending all three confluxes. Middle gets very little traffic so one man can easily defend it and chances are your third guy on the left will also be watching middle, so it's a cakewalk. I'm usually the guy at mid on my group and I'm easily able to defend my conflux as well as quickly help left or right if they need a a little support. So yeah, 3, 1, 2 is the best formation.