That's the thing, in all the times I've played this non-multiboxed I have never seen a character get focussed by all three, to the point they get hit by 3 shield-breakers, then get charged by all 3 at once, so I'm guessing theres something in the code that makes them pick a character and focus it you're all sat on top of each other (like only multiboxers will do). I wouldn't be surprised if they made it that way, just to stop boxers from farming abyss crystals, seeing as on normal this is probably the best place to do that.