Now THAT is super-impressive. You guys are definitely on top of the 3 man progression game… ;-) As you said, tanks are overrated.
For whatever reason, it never occurred to me to 3 man this boss without swapping. I knew that there was no agro dump (something which appeared to surprise you guys), but I had forgotten that Fearful is a largely-ignorable mechanic in story mode. It is somewhat amusing, btw, that having fewer people actually made certain parts of the fight *easier* on the healing (e.g. cleansing and the AoE circles).
I'm actually somewhat surprised that you couldn't 3 man the tanks. We 5 manned it the other day (not for the challenge really, but because one person really wanted to do SM EC and we couldn't be bothered to find more people), and the healing was a joke compared to T&Z. If the healer is dropping lightning between the tanks, the DPS could stand in front and only get hit by their respective cleaves. You'd definitely need a sentinel to "tank" Stormcaller though, since they'd have to jump over to Firebrand to kite the DD onto the other two. Maybe a Guardian would be better, so they could reset the jump CD and leap back immediately. Not sure.
Anyway, big congratulations!
Well, I can see a main problem with Firebrand : incinerate armor. Even if in SM a tank can easily take it without problem, the healer will have some issue on keeping both DPS alive.