Internally, memoir
does
\newcommand\@memmain@floats{%
\counterwithin{figure}{chapter}
\counterwithin{table}{chapter}
}
which means the figures and tables in the main-matter will be numbered subordinated to the chapter
counter; to override this, you can add to the preamble
\makeatletter
\renewcommand\@memmain@floats{%
\counterwithin{figure}{subsubsection}
\counterwithin{table}{subsubsection}
}
\makeatother
A complete example:
\documentclass{memoir}
\usepackage{amsmath}
\setsecnumdepth{subsubsection}
\maxtocdepth{subsubsection}
\numberwithin{figure}{subsubsection}
\numberwithin{table}{subsubsection}
\makeatletter
\renewcommand\@memmain@floats{%
\counterwithin{figure}{subsubsection}
\counterwithin{table}{subsubsection}
}
\makeatother
\begin{document}
\mainmatter
\chapter{Test chapter}
\vfill% just for the example
\section{Test section}
\subsection{Test subsection}
\subsubsection{Test subsubsection}
\begin{figure}
\caption{test}
\end{figure}
\end{document}
By the way, since memoir
internally uses chngcntr
, you can directly use \counterwithin
instead of \numberwithin
:
\counterwithin{figure}{subsubsection}
\counterwithin{table}{subsubsection}
As a side, personal note, I'd suggest you to reconsidering this numbering schema; having a string so long for the numbers is not very reader-friendly.