I'd suggest sticking with frost. Especially at your level with three mages, you could cast two frostbolts and a single mob is dead. If you were fire, maybe you could kill a mob with 3x pyros, but the cast is slow and you aren't speeding up the grind much at all. Plus, you get a lot more benefit for Aoe'ing with frost. With your team, it sounds like itd be fun to shield one mage (PW:S and Ice Barrier), gather up the mobs and bring them to two mages with blizzards ready to go and a holy nova. That would take care of mobs with more health than your AE+HN combo could handle quickly, plus probably be more effecient. So, I'd suggest stick with frost. I don't think you'd gain anything going fire over frost. =D