How to place three pages on one larger page?

Question

I seek advice how to place three pages on one larger page. I would like to place front cover page, spine and back cover page on one page. It is not obvious from the MWE, but the images on front / back cover should be placed in pair under each other. Pictures illustrated the desired placement (the whale should not be there :)). Thank you in advance.

Edit: The solution should preferably include the possibility to stretch / shrink the spine according to the real width of pages.

Here is my attempt but it places pages under each other.

\documentclass[8pt]{extbook}    
\usepackage[paperwidth=400mm, paperheight=280mm, top=0cm, headsep=0cm, bottom=0cm, left=0cm, right=0cm]{geometry}
\usepackage[icelandic, latin, czech]{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}     
\usepackage{tikz}
\usetikzlibrary{calc}
  \usepackage{color}
\usepackage{mwe}

%\usepackage{lscape} % for printing
\usepackage{pdflscape} %for viewing
\usepackage{pagecolor}
\usepackage{afterpage}    
\usepackage{adjustbox} % loads also graphicx
\usepackage%[showframe]
           {geometry}           

% Basic geometry of document
\geometry
  { layoutsize = {360mm,240mm}
  , layoutoffset = {2cm,2cm}
  , showcrop
  }

\graphicspath{%
  {/images/}}

% #################################################################### COLORS #

\definecolor{darkgreen}{rgb}{0.4, 0.01, 0.24}
\definecolor{title}{RGB}{16, 13, 32}

% #################################################################### #

\newcommand\makebookmarkfront{
 \begin{landscape}
\begin{center}
\vspace*{\fill}
 {\Huge\scshape\color{darkgreen}%
   \dictnameCZ\\\dictnameIS}
 \vspace*{\fill}
\end{center}
\end{landscape}
\clearpage
 }

\def\dictnameCZ{islandsko-český studijní slovník}
\def\dictnameIS{íslensk-tékknesk stúdentaorðabók}
 \renewcommand{\baselinestretch}{0.7}

 \newcommand\makebookmarkfrontd{
 \vspace*{0.3cm}%{\fill}
 \begin{figure}[ht]\centering%
 \setlength\fboxsep{0pt}\setlength\fboxrule{0pt}%
 \fbox{\includegraphics[width=4cm]{example-image-b}}
 \end{figure}

 \vspace*{0.90cm}%{\fill}

%\begin{minipage}{0.6\textwidth}

 \begin{center}\Huge\scshape\color{darkgreen}%
   islandsko-\\
   český\\
   studijní\\
   slovník
  \end{center}

    \vspace*{0.1cm}%{\fill}

   \begin{center}\Huge\scshape\color{darkgreen}%
   íslensk-\\
   tékknesk\\
   stúdenta-\\
   orðabók
   \end{center}

%\end{minipage}


\vspace*{10cm}%{\fill}

\begin{center}
{\color{darkgreen}www.hvalur.org}
\end{center}

 \vspace*{\fill}
\clearpage
 }


% #################################################################### COVERS #
\newcommand*{\frontcoverimages}{%
  example-image-b,
  example-image-b,
  example-image-b,
 example-image-b,
 example-image-b,
  example-image-b,
  example-image-b,
  example-image-b}

\newcommand*{\backcoverimages}{%
  example-image-a,
  example-image-a,
  example-image-a,
  example-image-a,
  example-image-a,
  example-image-a,
 example-image-a,
  example-image-a}

\newcommand\makecoverwith[1]{
 %\pagecolor{title}\afterpage\nopagecolor
 \begin{center}
 \vspace*{0.3cm}%{\fill}
 {\Huge\scshape\color{darkgreen}%
   \dictnameCZ\\\dictnameIS}
 \vspace*{1em}                        % I think this is necessary
 \begin{figure}[ht]\centering%
   \setlength\fboxsep{0pt}\setlength\fboxrule{0.5pt}%
   \foreach \file in #1{
     \fbox{\includegraphics[width=5.5cm]{\file}}}
 \end{figure}
 \vspace*{\fill}
 \end{center}
}


% ############################### Document
 \begin{document}
 \pagenumbering{gobble}
%    \pagecolor{title}
    \makecoverwith\frontcoverimages
    \makebookmarkfrontd{}
  \makecoverwith\backcoverimages
 \end{document}