http://elderscrollsonline.com/en/news/post/2013/09/19/developer-question-of-the-week-39

Ouch.. toughy this week:

Id go for the staff wielder since there wasnt specifed information on what type and i dont want them to get a spell off to find out. The bow wielder next and then the axe wielder since the cleave will be easy to avoid if any at all.

MMO Rule #1 kill the mage/healer first, so in this case I would kill the guy with the staff, then the bow with robes (robes are probably going to help mages almost exclusively so t is probably some sort of Arcane Archer. As you said, the warrior might be the heavy hitter of the three, but easier to avoid then ranged attackers.