I tend to roam alone as a hunter, but I hardly have seen 2/3 firemages which group together to stomp on a single character.
The worst I happened to see, to stay on topic, was 2 firemage and a priest ( premade ), but there the problem was the fact they were in premade ( they happened to win some fights, but they hardly cast any greater pyro ).
i have had enough of gpyro for one expac
As a DH I got two stuns and an interrupt. If a mage tries to cast through all of that then they will be dead.
However, I used my Rain From Above spell the other day and tabbed to the mage to see him casting greater pyroblast. Nothing you can do in that instance but kiss your own ass goodbye. Learned to never cast that ability with a fire mage nearby.