Arrange three figures using subcaption in two rows, horizontally centered with a caption for each figure

Question

I want to place three subfigures in one figure using the subcaption package. All three are equal in dimensions, and for each figure an own caption is required. In the first row there should be two figures, in the second row the third figure, centred horizontally.

The following MWE shows how far I got. Two lines, one caption per figure is working, I however cannot get the third figure centered.

\documentclass{scrartcl}
\usepackage{subcaption}
\usepackage{graphicx}
\usepackage{mwe}

\begin{document}

\begin{figure*}[t]
  \begin{subfigure}[b]{0.475\textwidth}
    \centering
    \includegraphics[width=\textwidth]{example-image-a}
    \caption[]%
    {{\small a cool description}}
    \label{fig:a}
  \end{subfigure}%
  \hfill
  \begin{subfigure}[b]{0.475\textwidth}
    \centering
    \includegraphics[width=\textwidth]{example-image-b}
    \caption[]%
    {{\small a cool description}}
    \label{fig:b}
  \end{subfigure}

  \begin{subfigure}[b]{0.475\textwidth}
    \centering
    \includegraphics[width=\textwidth]{example-image-c}
    \caption[]%
    {{\small a cool description}}
    \label{fig:c}
  \end{subfigure}
  \caption[ Global caption ]
  {\small Cool figure}
  \label{fig:test-figure}
\end{figure*}

\blindtext

\end{document}