How do I put my spine in the middle?

My spine right now it's on the left side of the white line that represents the center.

The white line doesn't represent the center, it's the fold mark between the spine and front cover of the 3 panel spread. Your export to PDF will be as a single spread

When you design a facing page book the spreads are folded on the center line and gathered into signatures.

@Willi Adelberger said: "It does not matter here, on which side of the line the spine is."

I agree very much on this.

Hi @Chris P. Bacon  ,

when done with a facing-pages document like yours simply add a page in the first spread dedicated for the spine of the book jacket.

It does not matter at all, if technically spoken, there is a "spine" at all ( non-facing pages document ) or if the technical spine ( facing pages document ) is at the left, at the right, between page 1 or page 2 or, if you add one, to page 3. In every case one could export all two or three pages by exporting the whole spread.

Perhaps one thing to add:

It would make very much sense, as Rob Day has showed, to add a page to the first spread that is meant for the "spine" of the book jacket. It would be easier to change its width with automatically moving elements on it when you do this in a facing-pages document.

Regards,
Uwe Laubender
( Adobe Community Professional )

Don't allow to shuffle pages.

You'll probably have to create separate file for the covers with the spine

Typically the pages inside your book don't need a spine.

And, is it possible to give the page number 3 a dynamic number, which would mean: the page number before the last page?

My content has to be provided in a separate PDF file, therefore I cannot sync the numberings between the 2.

But, there will never be a page number on the page that is marked with number 3, because both 2 and 3 will be advertisement pages, right on the back side of the cover pages.

Will that cause any confusion for the printer?