我该如何解决我的定心问题？

Question

我想将第三张图片居中。以下是代码和结果：

\begin{figure}[h]
\begin{subfigure}{0.6\linewidth}
  \includegraphics[width=0.7\textwidth, height=5 cm]{3_8_1_Under-and-Overexposure_Overexposure-Sample.jpg}
  \centering   
    \caption{A sample of Over-exposured Image}
  \label{fig:3_8_1}
  \centering
  \end{subfigure}
  \hfill
\begin{subfigure}{0.6\linewidth}
  \includegraphics[width=0.7\textwidth, height=5cm]{3_8_2_Under-and-Overexposure_Underexposure-Sample.jpg}
  \centering  
  \caption{A sample of Under-exposured Image}
   \label{fig:3_8_2}
  \centering  
   \end{subfigure}
    \begin{subfigure}{0.6\linewidth}
  \includegraphics[width=0.7\textwidth, height=5cm]{3_8_3_Under-and-Overexposure_Correct-Exposure-Sample-960x640.jpg}
  \centering  
  \caption{A sample of Well-exposured Image}
   \label{fig:3_8_3}
  \centering 
     \end{subfigure}
   \caption*{Source: https://photographylife.com/underexposure-and-overexposure-in-photography/}
 \caption{Samples for Over-exposured, Under-exposured and Well-exposured Images}  
    \end{figure}

Answer 1

在您的代码片段中，我删除s\centering内的所有 s ，在环境的开头subfigure添加并在最后一个之前插入空行：\centeringfiguresubfigure

\documentclass{article}
\usepackage[demo]{graphicx}
\usepackage{subcaption}

\begin{document}    
    \begin{figure}[h]
\centering
\begin{subfigure}{0.48\linewidth}% <-- changed
  \includegraphics[width=\linewidth, height=5 cm]{3_8_1_Under-and-Overexposure_Overexposure-Sample}
    \caption{A sample of Over-exposured Image}
  \label{fig:3_8_1}
  \end{subfigure}
  \hfil
\begin{subfigure}{0.48\linewidth}
  \includegraphics[width=\linewidth, height=5cm]{3_8_2_Under-and-Overexposure_Underexposure-Sample}
  \caption{A sample of Under-exposured Image}
   \label{fig:3_8_2}
   \end{subfigure}

    \begin{subfigure}{0.48\linewidth}
  \includegraphics[width=\linewidth, height=5cm]{3_8_3_Under-and-Overexposure_Correct-Exposure-Sample-960x640}
  \caption{A sample of Well-exposured Image}
   \label{fig:3_8_3}
     \end{subfigure}

\caption*{Source: https://photographylife.com/underexposure-and-overexposure-in-photography/}

\caption{Samples for Over-exposured, Under-exposured and Well-exposured Images}
    \end{figure}
 \end{document}

编辑：为了使标题和注释居中\caption*（我建议将其括在标题中），您需要在序言中添加（如果这在文档中的所有标题中都有效）或仅对此图有效的环境\captionsetup{justification=centering, font=small}的开头）：figure

\documentclass{article}
\usepackage{geometry}
\usepackage[demo]{graphicx}
\usepackage{subcaption}
\usepackage{caption}
    \captionsetup{justification=centering, font=small}
\usepackage{url}

\begin{document}    
    \begin{figure}[h]
    \centering
\begin{subfigure}{0.48\linewidth}% <-- changed
  \includegraphics[width=\linewidth, height=5 cm]{3_8_1_Under-and-Overexposure_Overexposure-Sample}
    \caption{A sample of Over-exposured Image}
  \label{fig:3_8_1}
  \end{subfigure}
  \hfil
\begin{subfigure}{0.48\linewidth}
  \includegraphics[width=\linewidth, height=5cm]{3_8_2_Under-and-Overexposure_Underexposure-Sample}
  \caption{A sample of Under-exposured Image}
   \label{fig:3_8_2}
   \end{subfigure}

\medskip
    \begin{subfigure}{0.48\linewidth}
  \includegraphics[width=\linewidth, height=5cm]{3_8_3_Under-and-Overexposure_Correct-Exposure-Sample-960x640}
  \caption{A sample of Well-exposured Image}
   \label{fig:3_8_3}
     \end{subfigure}

\caption*{}

\caption[Samples for Over-exposured, Under-exposured and Well-exposured Images]
        {Samples for Over-exposured, Under-exposured and Well-exposured Images.\\ Source: \url{https://photographylife.com/underexposure-and-overexposure-in-photography/}}
    \end{figure}
 \end{document}