Open the starting book:
Add Existing: <second epub>
Add Existing: <third epub>
(Wash, spin: REPEAT)
Note: each additional EPUB will ADD the Stylesheet (renumbered if it has an name that is already used).
If they ANY are the same, SELECT (in the Book Browser) JUST those sections: Right-click: Link stylesheets: (select only
ONE VERSION of the duplicated sheets . Any previous sheets will be de-referenced in those files. Be sure to re-link all necessary sheets if you use multiple sheets per section. If a section started out with 3 sheets used, it still needs 3 sheets)
Or just Search and replace the old sheet name with the current .
Run flight crew: It should report
Unused CSS, Delete those