Why don't three-phase transformers use toroidal cores?
To answer your comment on three-phase torroidal:

Because it seems, per wikipedia: Toroidal inductors and transformers, that the design should be superior. But I see no mention of three-phase usage, only single-phase.


Figure 1. 3-phase transformer flux. Source: NPTEL.
In a three-phase transformer each primary and secondary pair are wound on the same &quot;limb&quot; or &quot;branch&quot;. With the 120° phase difference on each branch the flux on one branch can always find a path on the other two so that there is always a flux circuit. For example, when the red phase (Fig. 1) is max upwards the yellow and blue will be 0.5 downwards.
This arrangement is not possible on a standard torroidal transformer.

Figure 2. A three-phase &quot;torroid&quot; would have to look something like this. (Image mine.)