My main team has always 2 tanked dark shaman on normal and never had much trouble with it, even learning the fight, but I have done the three tank method in heroic as well as normal/flex when helping out lesser geared (also bad) people try to progress through the stuff.
Using three tanks makes the entire fight trivial really. Is it required? Of course not, the fight isn't that bad with two tanks. But does it make it a hell of a lot easier? Yes it does. I can't tell you how many groups i've seen that were unable to make any progress on this fight at all 2 tanking it that 1-shot it the first time they 3 tanked it.
All it really requires is the haromm tanks placing walls so they are well controlled. Personally I tank him in such a way that each wall he drops is right next to and running parallel with the walls inside the room. Then you just shift over to the next wall, theres like 6 of these walls, it's very easy to do. Then just try to minimize the green shit people stand in which is as simple is spreading out to be honest.
Easy peasy fight.
The random resets are quite annoying, i've seen them happen about 4 times now I think, but funny enough i've never seen them randomly reset during a 3 tank fight, usually they randomly despawn/reset when 2 tanking. Really wish blizz could make shit that worked right.