\usepackage{subfig}和\subfloat{}

需要注意的是使用了\newline来另起一行。

......
\usepackage{ graphicx}
\usepackage{ subfig}
......
\begin{ document}
\begin{ figure*}[ht]
  \subfloat[img 1]{ \includegraphics[width=0.3\textwidth]{ 1.jpg}}
 \hfill 	
  \subfloat[img 2]{ \includegraphics[width=0.3\textwidth]{ 2.jpg}}
 \hfill	
  \subfloat[img 3]{ \includegraphics[width=0.3\textwidth]{ 3.jpg}}
  \newline
  \subfloat[img 4]{ \includegraphics[width=0.3\textwidth]{ 1.jpg}}
 \hfill 	
  \subfloat[img 5]{ \includegraphics[width=0.3\textwidth]{ 2.jpg}}
 \hfill	
  \subfloat[img 6]{ \includegraphics[width=0.3\textwidth]{ 3.jpg}}
\caption{ Caption}
\end{ figure*}
......
\end{ document}

效果：

\usepackge{subcaption}和\subfigure{}

需要注意的是使用了\newline来另起一行。

......
\usepackage{ graphicx}
\usepackage{ subcaption}
......
\begin{ document}
......
\begin{ figure*}[ht]
\begin{ subfigure}{ .3\textwidth}
  \centering
  % include first image
  \includegraphics[width=.8\linewidth]{ 1.jpg}
  \caption{ Put your sub-caption here}
  \label{ fig:sub-first}
\end{ subfigure}
\hfil
\begin{ subfigure}{ .3\textwidth}
  \centering
  % include second image
  \includegraphics[width=.8\linewidth]{ 2.jpg}
  \caption{ Put your sub-caption here}
  \label{ fig:sub-second}
\end{ subfigure}
\hfil
\begin{ subfigure}{ .3\textwidth}
  \centering
  % include second image
  \includegraphics[width=.8\linewidth]{ 3.jpg}
  \caption{ Put your sub-caption here}
  \label{ fig:sub-second}
\end{ subfigure}
\newline
\begin{ subfigure}{ .3\textwidth}
  \centering
  % include first image
  \includegraphics[width=.8\linewidth]{ 1.jpg}
  \caption{ Put your sub-caption here}
  \label{ fig:sub-first}
\end{ subfigure}
\hfil
\begin{ subfigure}{ .3\textwidth}
  \centering
  % include second image
  \includegraphics[width=.8\linewidth]{ 2.jpg}
  \caption{ Put your sub-caption here}
  \label{ fig:sub-second}
\end{ subfigure}
\hfil
\begin{ subfigure}{ .3\textwidth}
  \centering
  % include second image
  \includegraphics[width=.8\linewidth]{ 3.jpg}
  \caption{ Put your sub-caption here}
  \label{ fig:sub-second}
\end{ subfigure}
\caption{ Put your caption here}
\label{ fig:fig}
\end{ figure*}
......
\end{ document}

效果：

\usepackage{subfigure}和\subfigure{\minipage{}}

这种方法在之前的博文LaTeX subfigure和minipage的使用中也有介绍过。

......
\usepackage{ graphicx}
\usepackage{ subfigure}
......
\begin{ document}
......
\begin{ figure*}
\centering
\subfigure[Caption a]{ 
\begin{ minipage}[b]{ 0.23\textwidth}
\includegraphics[width=1\linewidth]{ 1.jpg}\vspace{ 4pt}
\includegraphics[width=1\linewidth]{ 2.jpg}\vspace{ 4pt}
\includegraphics[width=1\linewidth]{ 3.jpg}
\end{ minipage}}
\subfigure[Caption b]{ 
\begin{ minipage}[b]{ 0.23\textwidth}
\includegraphics[width=1\linewidth]{ 1.jpg}\vspace{ 4pt}
\includegraphics[width=1\linewidth]{ 2.jpg}\vspace{ 4pt}
\includegraphics[width=1\linewidth]{ 3.jpg}
\end{ minipage}}
\subfigure[Caption c]{ 
\begin{ minipage}[b]{ 0.23\textwidth}
\includegraphics[width=1\linewidth]{ 1.jpg}\vspace{ 4pt}
\includegraphics[width=1\linewidth]{ 2.jpg}\vspace{ 4pt}
\includegraphics[width=1\linewidth]{ 3.jpg}
\end{ minipage}}
\subfigure[Caption d]{ 
\begin{ minipage}[b]{ 0.23\textwidth}
\includegraphics[width=1\linewidth]{ 1.jpg}\vspace{ 4pt}
\includegraphics[width=1\linewidth]{ 2.jpg}\vspace{ 4pt}
\includegraphics[width=1\linewidth]{ 3.jpg}
\end{ minipage}}
\caption{ description of figure}
\end{ figure*}
......
\end{ document}

效果：

对比之下，实现相同的效果，第一种\subfloat会更简洁。第三种包subfigure官方声明被弃用了。

三个包subcaption会更新功能更强大。具体需要进一步探索。

参考资料：

How to add subfigure in Latex