I should probably note that I don't have a strong opinion about including or not including a t3 mass fab command in the game.
The efficiency difference is small, so even if one or the other is 'better', it's not a huge deal either way.
Also both of the above options are less efficient than just building a t2 mass fab (3.0e-3 mps/m, at worst), which is also much more flexible imo. As such, I can't remember the last time I even built a t3 mass fab, hence the command probably wont affect my gameplay at all.
My only concern is that if we add a command like that in the game, we basically say that using it is "correct" as otherwise, why would it be there?