How to format/change three part header in Pages (as set by default)?

They're called Headers and Footers. As the Pages Help says: You can add headers and footers — fields at the top and bottom of a page that can include text, images, page numbers, the page count and the date and time — to your document and then set where you want them to appear.

Note: Before you begin, find out whether you’re working in a word processing or page layout document. There are some differences in how you format headers and footers for each type of document.

If you have no need for them you can just ignore them or find a template that doesn't contain them.

The tripartite header/footers appear like table cells but are not, and do not offer alignment features as one expects in body text, or other containers offering text input.

One can suppress the header and footer fields in the Document panel, and then replace them with your choice of a single-row table with columns to suit your header goals. This offers cell alignment functionality. And ultimately, you can save this new header/footer object via the Arrange panel > Section Layouts > Move Objects to Section Layout. I would only recommend doing this if your destination document is a PDF.