I had some trouble with this one too, but sorta figured it out. You wanna start off with HoTs, zero DPS, and a fair whack of AOE threat. Glyphed Maul, Berserk, and Swipe them before you start DPS, and before the fight starts spreading out. Once you start killin', you'll want to keep one or both of the other champs interested with an occasional Swipe. If you load up on threat at the start, it's not such a big deal as the fight progresses, i.e. by the time you're starting to really lose threat, you're finished with their friend and can get properly in their face anyway.

Now that I've got threat worked out, my trouble with this fight is that I have so few ways of dealing with healing waves. Two nights ago, I left the pally to kill last, and he essentially tanked my entire party for ten minutes. I could only Bash him so often, he couldn't kill me, and I didn't have the mana to just burn him down through the heals. So, between Innervate cooldowns, I ended up just letting him run himself out of mana by damaging him a little and letting him heal, damaging him a little, rinse and repeat. Then when I thought it couldn't be more boring ... quick, pop Innervate and go all out. It was ridiculous.