Impozycja broszury, pdf

A cover is a separate file. 
Create a 3 page document with facing pages. 
put all 3 pages into a single spread. It can be done via the page panel menu command to keep pages together. 

Make the margins of the spine page 0mm

Change the width of the spine page to the final width, here 4mm. 

you could also make a single page in the final size, bit above described steps gove you more flexibility, if the width of the spine will will change if you decide to print differently.

https://creativepro.com/set-up-a-book-cover/

@Clever_wanderer6329

A jaki rodzaj impozycji?

Jak to będzie drukowane i wykańczane? 

Szyte, klejone, miękka czy twarda oprawa?

Zadałeś zbyt ogólne pytanie. 

Imposition is the printer's responsibility, not yours.