There's an old saying "three is a crowd" - Not enough tanks?

[Alahra]

Parses have proven all three tanks coming really close to one another in terms of DPS with WAR being slightly ahead. The difference is ~8%.

8% isn't insignificant, though. Honestly, it's probably less about the DPS that each can put out and more about the fact that WAR is the only OT with innate TP recovery options. That gives them more longevity, which is pretty important, I'd say. Add in the fact that WARs have naturally higher DPS, and there's really not a good reason to take one of the other two for OTing.