Page 3 in the footer on page 3 is 1 not 3 and when I change it too 3 in footer page 1 in the footer gets changed too page 3.
How did I cause this or do wrong?
I just typed the number in the footer and correct number for that page,
which is exactly all that should be needed.
Maybe your document was created and/or edited in Microsoft Word. The Word can not handle the Page sytyles, therefore every page will have individual Page Style in the LibreOffice, and maybe they will have a restarted page numbering.
Can you upload an ODF type sample file here?
In the menu, click Insert - Page number and choose the position and type