Includepdf as a figure

As already said by others in the comments you need to use \includegraphics directly because \includepdf uses a page of its own. You can use a tabular to get the 2x2 layout and use the page=<number> key to select the page:

\documentclass{article}

\usepackage{graphicx}
\usepackage{blindtext}

\begin{document}
\blindtext

\begin{figure}[h]
   \centering
   \begin{tabular}{@{}c@{\hspace{.5cm}}c@{}}
       \includegraphics[page=1,width=.45\textwidth]{somemultipagepdf} & 
       \includegraphics[page=2,width=.45\textwidth]{somemultipagepdf} \\[.5cm]
       \includegraphics[page=3,width=.45\textwidth]{somemultipagepdf} \\
   \end{tabular}
 \caption{Test}
 \label{fig:Test}
\end{figure}

\blindtext
\end{document}

pdfpages typically inserts page breaks before inserting content, causing problems if you want to have an arrangement if pages inside a figure environment. Instead, use the page key-value of \includegraphics itself to insert the contents on a page-by-page basis. Something like this:

\begin{figure}[ht]
  \centering
  \includegraphics[page=1,width=.3\textwidth]{test}\hspace*{.25\textwidth}%
  \includegraphics[page=3,width=.3\textwidth]{test}

  \includegraphics[page=2,width=.3\textwidth]{test}
  \caption{Test}
  \label{fig:Test}
\end{figure}

If you want the last "image" (page=2) to be shown in a nup=2x2 fashion, you could add \hspace*{.55\textwidth} after it, otherwise it will be horizontally centred below the two "images" (page=1 and page=3) above it.

Of course, you can play with the width adjustment to suit your liking.