We have Robe of Earth, Robe of Flame, Robe of Cold. They are all self-buffs, and they sound great. Since I only got to lvl 14 during open beta, I only saw Robe of Earth and never had more than one of these spells, otherwise I would know this for myself...

Can you 'wear' multiple 'robes' at the same time, or does applying a new one cancel out the previous one?

Which ever robe you activate cuts off the other one, so only one.

I had a bad feeling you were going to say that

I wonder how unballancing it would actually be if we could have all 3 of them active simultaneously. At their strongest versions I can see, you would have:
For 30 min
each time an enemy within 30m attacks you, you reflect 40 damage to the enemy and slow down its movement speed for 4 sec. (Robe of Cold 2)
AND
your Magic Boost increases by 102 (Robe of Flame 3)
AND
increase your Natural Mana Treatment by 56, Fire Resistance by 35, and Water Resistance by 35 (Robe of Earth 4)

I mean seriously, we still only have cloth with crap for damage mitigation, which is fine. Improved fire and water resistance helps us out against magic attacks and a few physical attack specials our opponents would use agaisnt us but certainly not a going to take away our Glass Cannon crown. Adding the magic boost at the same time is not going to hurt. The reflected 40 damage on top of that is nice but i think a lvl 1 Mage does more damage with their spellbook (and that isnt exactly much damage even then).

My point being that even at their strongest, the combined effect should not be overpowering when considdering the comparitive power of everything else going on. Nice self buffs to have, but i just dont see why they have to be mutually exclusive. Not that the Devs will care about such a piddly little issue, because it is small in the grand scheme of things.