大子图问题

Question

我有以下代码：

    \subsection{Grouped Test Images with their Histograms}
\subsubsection*{Portrait}
\begin{figure}[h]
\begin{subfigure}{\linewidth}
  \includegraphics[width=\textwidth, height=6cm]{einstein_image&histogram.jpg}
  \centering
  \caption{Einstein Image and Histogram}
  \label{fig:3_9_1}
  \caption*{Source: https://processing.org/tutorials/color/}
  \end{subfigure}
\hfill
\begin{subfigure}{\linewidth}
  \includegraphics[width=\textwidth, height=6cm]{lena_image&histogram.jpg}
  \centering
 \caption{Lena Image and Histogram}
  \label{fig:3_9_2}
  %if there are any # $ % & ~ _ ^ \ { } in your URLs, Latex will not be nice to you. 
  %http://stack_overflow.com/~foo%20bar#link would be
  %http://stack\_overflow.com/\~foo\%20bar\#link
  \caption*{Source: http://www.imageprocessingplace.com/root\_files\_V3/image\_databases.htm}
 \end{subfigure}
 \hfill
\begin{subfigure}{\linewidth}
  \includegraphics[width=\textwidth, height=6cm]{woman_image&histogram.jpg}
  \centering
  \caption{Woman Dark Hair Image and Histogram}
  \label{fig:3_9_3}
  \caption*{Source: http://www.imageprocessingplace.com/root\_files\_V3/image\_databases.htm}
 \end{subfigure}
 \end{figure}

问题是我发现我的子图位于文档末尾。我该如何解决这个问题？