Tertiary MVA of Autotransformer

What did the transformers' manufacturer say during the telephone calls on this topic?

https://www.eng-tips.com/viewthread.cfm?qid=125717

Absolutely no.

Usually transformers have one winding, not even a second, so your question is very confusing.

Each winding should have a power rating. Provide the nameplate if you do not know how to interpret it. 1/3rd is very likely to be wrong.

Are you talking about a custom transformer which includes an additional winding for local utility power purposes? If so 1/3 seems way too high to me. 50-500kVA perhaps.

What's the additional winding actually being used for (or is this all a hypothetical situation)?

A tertiary winding on a large power autotransformer is often used for auxiliary power requirements such as station service. That is not uncommon. The rating of that tertiary winding should be shown on the transformer's nameplate, with an MVA or KVA rating for each winding. If the nameplate doesn't show it (it should!), the manufacturer will be able to tell you.