\begin{figure}
\begin{subfigure}{0.2\textwidth}
  \includegraphics[width=0.65\textwidth]{Capture.PNG}
  \caption{1}
  \label{fig:fig1}
\end{subfigure}
+
\begin{subfigure}{0.2\textwidth}
  \includegraphics[width=0.65\textwidth]{Capture.PNG}
  \caption{2}
  \label{fig:fig2}
\end{subfigure}
=
\begin{subfigure}{0.2\textwidth}
  \includegraphics[width=0.65\textwidth]{Capture.PNG}
  \caption{3}
  \label{fig:fig3}
\end{subfigure}
\caption{plots of....}
\label{fig:fig}
\end{figure}