I'll fire up T3 tonight and see how it goes though.
I did it with 500k health and 800 res (Should mention I run the superstition passive which is 20% elemental reduction) all but with 40k regen. The jump to T4 was survivable as well but I couldn't kill anything in a reasonable amount of time. Did an Act I run of bounties on T4 and Skeleton King took about 15 minutes to kill.