How to change one header to 3-column header?

When you open a Pages document created prior to Fall 2013, or a Word document, these originally only contain a single header/footer segment and that will be preserved when opened in Pages versions after Fall 2013 (including Pages v12.1).

You cannot change this without opening those documents in one Pages window, and then creating a new blank Pages document where you copy/paste the old content into the new Pages document with its tripartite header/footer.

The three header boxes are supplied automatically. You can place running text or page number calculator into any or all of the three boxes.

Text in the left box will be aligned to the left margin of that box.

Text in the centre box will be aligned with the centre of the box.

Text placed in the right box will be aligned to the right margin of the box.

Text placed in any of the boxes will overflow into the adjacent box if it is too long to fit into the box where it was entered, AND if the box into which it would flow is otherwise empty.

A three-part header is more clearly described as a set of three header items, each of which fits into a single header box.

Headers (and Footers, which follow the same rules, do not have 'columns'. The boxes into which they fit will contain only a single 'row' of text.

Regards,

Barry