I want to group images and assign the same sub-figure number in a figure consisting of several such groups. Please have a look at the following image to understand my question properly.

Two options, but the general idea is the same: to use one subfigure environment (requires subcaption) or \subfloat command (requires subfig) for each group.
Here's one option using the powerful subcaption package:
\documentclass{article}
\usepackage{subcaption}
\usepackage{graphicx}
\begin{document}
\begin{figure}
\begin{subfigure}{\linewidth}
\includegraphics[width=.3\linewidth]{example-image-a}\hfill
\includegraphics[width=.3\linewidth]{example-image-a}\hfill
\includegraphics[width=.3\linewidth]{example-image-a}
\caption{}
\end{subfigure}\par\medskip
\begin{subfigure}{\linewidth}
\includegraphics[width=.3\linewidth]{example-image-b}\hfill
\includegraphics[width=.3\linewidth]{example-image-b}\hfill
\includegraphics[width=.3\linewidth]{example-image-b}
\caption{}
\end{subfigure}\par\medskip
\begin{subfigure}{\linewidth}
\includegraphics[width=.3\linewidth]{example-image-c}\hfill
\includegraphics[width=.3\linewidth]{example-image-c}\hfill
\includegraphics[width=.3\linewidth]{example-image-c}
\caption{}
\end{subfigure}
\caption{Some grouped images}
\end{figure}
\end{document}
The output:

And now with the help of the subfig package:
\documentclass{article}
\usepackage{subfig}
\usepackage{graphicx}
\begin{document}
\begin{figure}
\subfloat[]{%
\begin{minipage}{\linewidth}
\includegraphics[width=.3\linewidth]{example-image-a}\hfill
\includegraphics[width=.3\linewidth]{example-image-a}\hfill
\includegraphics[width=.3\linewidth]{example-image-a}%
\end{minipage}%
}\par
\subfloat[]{%
\begin{minipage}{\linewidth}
\includegraphics[width=.3\linewidth]{example-image-b}\hfill
\includegraphics[width=.3\linewidth]{example-image-b}\hfill
\includegraphics[width=.3\linewidth]{example-image-b}%
\end{minipage}%
}\par
\subfloat[]{%
\begin{minipage}{\linewidth}
\includegraphics[width=.3\linewidth]{example-image-c}\hfill
\includegraphics[width=.3\linewidth]{example-image-c}\hfill
\includegraphics[width=.3\linewidth]{example-image-c}%
\end{minipage}%
}
\caption{Some grouped images}
\end{figure}
\end{document}
The output:

Another option might be to use the floatrow package.
try this
\begin{figure}{h}
\subfigure[caption 1]{\label{...}
\includegraphics{img1-a}\hspace{1em}
\includegraphics{img1-b}\hspace{1em}
\includegraphics{img1-c}
}
subfigure[caption 2]{\label{...}
\includegraphics{img2-a}\hspace{1em}
\includegraphics{img2-b}\hspace{1em}
\includegraphics{img2-c}
}
\caption{}
\label{}
\end{figure}
subfigure package which is obsolete and shouldn't be used anymore.
– Gonzalo Medina
Jun 11 '15 at 22:30
h as placement specifier is a recipe for disaster and the placement specifier goes as optional argument for figure and your code snippet uses it as mandatory argument.
– Gonzalo Medina
Jun 11 '15 at 22:41
\includegraphics{...}for each of the groups withsubfigthen? – Jun 11 '15 at 21:08