The team your describing is called EuroComp. Which was a very strong counter to the ever popular 2345 and 2346 setups that reigned supreme in all of Season 1 and 2.
I'd say I've mastered the technique to beating them as one of the best EuroComps in the game I've beaten 6-0, they are sponsored and have some pretty famous names on the team - Affix and Azazael.

First this team will lock you down, it's a given but you need to prevent this as much as possible.
Second the Mage on their team is the only threat. If you put pressure on him he will have to stay defensive and shouldn't be able to throw frostbolts on you. If you die from a Cheat Death Rogue or a SL/SL lock, then you've got bigger things to worry about
Best target is the Mage followed by the Priest, if you take out the Mage it's pretty much over for them, just outlast their petty damage and pop another target.

To note Triple Melee DPS teams are the counter to the EuroComp and many other teams. The counter to the Triple Melee dps is a Tri healer team. Pretty much 5v5 Arenas are Rock/Paper/Scissors at it's finest... except for 4x Shaman and a healer, we have no counter