\documentclass[12pt,a4paper]{report}
\usepackage{graphicx}
\usepackage{caption}
\usepackage{subfig}

\begin{document}

\begin{figure}[!ht]
\centering 
    \subfloat[A sub-figure]{%
      \includegraphics[width=0.45\textwidth]{example-image-a}
    }
    \\
    \subfloat[B sub-figure]{%
      \includegraphics[width=0.45\textwidth]{example-image-b}
    }
    \hfill 
    \subfloat[C sub-figure]{%
      \includegraphics[width=0.45\textwidth]{example-image-C}
    }
    \caption{Dummy figure}
    \label{fig:dummy}
  \end{figure}

\end{document}