There are two ways to play this depending on the Rogues' specs, but I will go through the most basic one first. Kill the Mage. Don't waste time sheeping the Rogue because in most games the match will nearly be over by the point you can get a decent duration sheep off on a competent Rogue with a competent Priest; sheep the Piest to start. Swap to the Rogue later in the game if you know his Vanish/Cloak of Shadows/Trinket/Priest are on DR. The Priest should be trying to Fear the Rogue or Priest situationally; make your own judgment here. If you can get an early Counterspell on the other Mage's Frost or Arcane, do it, but save a CS for the Priest later in the game (later being about 20 seconds~). If he exposes his Frost tree while low on HP though, CS it and kill him. The Priest's number one priority should be Mass Dispelling Ice Block. If your Mage is at 1% and has just blocked, and the other teams Mage has just blocked too, Pain Suppress him and go for the MD first. (This is just an example of how important getting your MD off fast is). The winning team will be the one that more skillfully spammed damage on the other team's Mage and the Priest that got the MD off.

The other way works if the other team's Rogue is Mutilate or 41 Combat. Kill the Rogue, they'll more than likely be on your Mage while theirs is left free, so you interrupting the Mage is alot more important here; Fear him, Poly him, CS him always, and whatever you do, stop him from Frostbolting or you will probably lose. On the other hand it's much easier for your Mage to kite a Rogue who has 1 Sprint, 1 Vanish, 1 cloak and nothing else to get Crippling off him who's also being constantly stunned while only having one Evasion and no Cheat Death. Until you get him decently low don't bother CCing the Priest, but when he's at sub 70% or so with all his cooldowns burned, start CCing the Priest instead of the Mage and go for a kill.