The principle of bucking or boosting involves connecting the primary and secondary windings of each of the transformers as an auto-transformer.

They would be series-aiding for boost operation, and series-opposing for bucking.
This is a single phase example-

[Linked Image from electric-ed.com]

There are many different three phase connections. You should follow the manufacturers recommendations.
Here is a typical example of how it could be done.

[Linked Image from electric-ed.com]

Ed