Three Phase Toroidal Transformer Design

Maybe three separate toroids, yes.

Or one could use one transformer for each phase, perhaps?

You might examine, what is left of flux in the core when all phases have a similar (same value) load and same cos. phi. Vectors can show this. You could find some similarities with the current through a zero (nul- neutral) in a Star (Wye) 3 phase net. Maybe, perhaps, probably when the balance is gone? Cores of 3 phase transformers have separate magnetic lanes. They can mould multiphase for ferrite too, but winding your coils will be a much bigger challenge as with single toroids.

Probably not as a standard torroid... at least not while acceptably small (physically and financially).

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Perhaps if your shape was something like the following but with a connection each at each place the core crosses over itself....

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I do not believe that this is possible with a single toroid. There is only one magnetic field path in a toroid. I believe that a three phase transformer requires three magnetic field paths. Adding one more hole to the topology provides three manifold paths that will allow for three magnetic field paths.

If somebody knows otherwise, I'd love to hear how this can be done.

It can probably be accomplished with an over-sized toroid with shorted coils wound between each phase, a-la shaded pole.

How about a Scot T transformer - 2cores.

The common practice is three separate toroidal cores. This is done both for ease of manufacture and cost as you use the same single phase toroidal transformer design and parts, just three of them.

I am not aware of any three phase toroidal transformers using a single toroid. Doesn't mean it might not be possible, but I cannot see any price or other advantages if it were.

A simple toroid will not work, but in a way the old shell-form transformer cores like Westinghouse used could be considered a form of 3 phase toroid. This simply has a straight section where the winding is. The core design allows for the magnetic coupling of the 3 phases. A shell form transformer has some advantages but is very expensive to manufacture. A 5 legged core is simpler and is usually used if those advantages are needed.