begin{figure}[ht!]

begin{subfigure}[t]{.5textwidth}

centering

begin{subfigure}[t]{.5textwidth}

centering

includegraphics[width=.6linewidth]{figures/image1.pdf}

   caption{a}

end{subfigure}%

quad

begin{subfigure}[t]{.5textwidth}

centering

includegraphics[width=.6linewidth]{figures/image2.pdf}

  caption{b}.}

  end{subfigure}%

quad

begin{subfigure}[t]{.5textwidth}

centering

includegraphics[width=.6linewidth]{figures/image3.pdf}

  caption{c}.}

  end{subfigure}%

caption{caption}

end{figure}