Hi, I've put from Page Layout three columns on a page, how can I get back to one column on the next blank page ? (So that just the first one has 3 - if I set it back to one, the first page changes too)

Thanks a lot!

To do that, you need to use a 'Next Page' Section break and change the page layout for the new Section back to one column.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]

Supplementing Paul's answer...
Columns are a section property. You can change them within a page or for a new page. Note that if you add text on the page with columns and it flows onto another page, that page will also have the columns.

Sections / Columns in Microsoft Word 2007-2013