In my opinion, the only way for DK/rogue/priest to beat RMP, is training the priest non-stop cheap WoW Gold.  Any smart RMP will open on your DK immediately to prevent him from getting to the priest right away. What I do is CS -> disarm the rogue, blind him, do whatever I can to buy my DK time to grip the priest, from there unload all of our CDs and drop the priest first 30 seconds of the game.  It is called die-by-the-sword because win or lose; we force a conclusion fast and avoid going late game against RMP as cleave at all costs.

Serennia-sama claims that you can split DPS between rogue/priest and for me that is as silly as splitting DPS as rogue/rogue/druid against RMP. You are trying to play control when you have very little usable CCs, when the RMP is the epitome of control. Frost mage is the ultimate play maker and an elite mage like Pookz will have no problem whatsoever slowing down one of your DPS while nuking down the other. Since he will be left open most of the times, all it takes is a couple of hard casts and a blanket counter spell and you are in kill range. Sheep->blind the priest, nova->deep freeze the DK, and nuke the rogue; it is all text books about cheap WoW Gold.

Yes, you can try to focus fire the rogue but with cloak of shadow glyph + double evasion, and mage/priest free to do whatever they want, other than lucky bursts during stuns (which their rogue can trinket since you have disease on him to prevent blind, their priest can pain suppress, their mage can CC one of your melees if not both to stop the damage), they will kill your rogue quicker more often than not. The correct play is to have your rogue peel their rogue, grip over the priest then unload everything on him to force a quick conclusion to cheap WoW Gold.