Logo on the bottom of title page - beamer - metropolis theme

You could define your own title page with the titlegraphic at the bottom:

\documentclass{beamer}
\usetheme{metropolis}

\titlegraphic{%
    \includegraphics[width=.2\textwidth]{example-image-a}\hfill
  \includegraphics[width=.2\textwidth]{example-image-b}\hfill
  \includegraphics[width=.2\textwidth]{example-image-c}
}

\makeatletter
\setbeamertemplate{title page}{
  \begin{minipage}[b][\paperheight]{\textwidth}
    \vfill%
    \ifx\inserttitle\@empty\else\usebeamertemplate*{title}\fi
    \ifx\insertsubtitle\@empty\else\usebeamertemplate*{subtitle}\fi
    \usebeamertemplate*{title separator}
    \ifx\beamer@shortauthor\@empty\else\usebeamertemplate*{author}\fi
    \ifx\insertdate\@empty\else\usebeamertemplate*{date}\fi
    \ifx\insertinstitute\@empty\else\usebeamertemplate*{institute}\fi
    \vfill
    \ifx\inserttitlegraphic\@empty\else\inserttitlegraphic\fi
    \vspace*{1cm}
  \end{minipage}
}
\makeatother

\begin{document}

\begin{frame}
    \titlepage
\end{frame} 

\end{document}

you can always include anything to any frame without defining new templates with package like eso-pic or tikz.

\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{positioning}
\usetheme{metropolis}
\begin{document}
\title{My presentation}
\author{Me}
\begin{frame}
\maketitle
\begin{tikzpicture}[overlay, remember picture]
\node[above right=1cm and .8cm of current page.south west] {\includegraphics[width=2cm]{example-image}};
\node[above =1cm of current page.south] {\includegraphics[width=2cm]{example-image}};
\node[above left=1cm and .8cm of current page.south east] {\includegraphics[width=2cm]{example-image}};
\end{tikzpicture}
\end{frame}
\end{document}