How to align these images?

Question

So I was inserting images like this in a sub-images and the last image is not coming as a whole. How to do this? Any help would be appreciated.

\usepackage{graphicx}
\usepackage{authblk}
\usepackage{float}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{algorithm}
\usepackage{amsmath}

\begin{figure}
 \centering
 \begin{subfigure}[b]{0.5\textwidth}
     \centering
     \includegraphics[width=\textwidth]{histo1.jpg}
     \caption{}
     \label{fig:y equals x}
 \end{subfigure}
 \hfill
 \begin{subfigure}[b]{0.5\textwidth}
     \centering
     \includegraphics[width=\textwidth]{histo2.jpg}
     \caption{}
     \label{fig:three sin x}
 \end{subfigure}
 \hfill
 \begin{subfigure}[b]{0.5\textwidth}
     \centering
     \includegraphics[width=\textwidth]{histo3.jpg}
     \caption{}
     \label{fig:five over x}
 \end{subfigure}
    \caption{ Histogram Plot of (a) Input Image, (b) Encrypted Image and (c) Decrypted Image }
    \label{fig:three graphs}
\end{figure}

Answer 1

You can have the first two images side by side, but obviously not the last one.

The problem is that you're inserting a space between the subfigures, so the first two cannot fit in a line. Look for %<--- to see where the space was.

For the third picture, it's better to have a paragraph break and some vertical space.

\documentclass{article}

\usepackage{graphicx}
\usepackage{authblk}
\usepackage{float}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{algorithm}
\usepackage{amsmath}

\begin{document}

\begin{figure}
 \centering
 \begin{subfigure}[b]{0.5\textwidth}
     \centering
     \includegraphics[width=\textwidth]{histo1.jpg}
     \caption{}
     \label{fig:y equals x}
 \end{subfigure}%<---
 \hfill
 \begin{subfigure}[b]{0.5\textwidth}
     \centering
     \includegraphics[width=\textwidth]{histo2.jpg}
     \caption{}
     \label{fig:three sin x}
 \end{subfigure}

\bigskip

 \begin{subfigure}[b]{0.5\textwidth}
     \centering
     \includegraphics[width=\textwidth]{histo3.jpg}
     \caption{}
     \label{fig:five over x}
 \end{subfigure}
    \caption{ Histogram Plot of (a) Input Image, (b) Encrypted Image and (c) Decrypted Image }
    \label{fig:three graphs}
\end{figure}

\end{document}

If you want the three pictures all on one line, use the textwidth divided by 3.

\documentclass{article}

\usepackage{graphicx}
\usepackage{authblk}
\usepackage{float}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{algorithm}
\usepackage{amsmath}

\begin{document}

\begin{figure}
 \centering
 \begin{subfigure}[b]{0.33333\textwidth}
     \centering
     \includegraphics[width=\textwidth]{histo1.jpg}
     \caption{}
     \label{fig:y equals x}
 \end{subfigure}%<---
\hfill
 \begin{subfigure}[b]{0.33333\textwidth}
     \centering
     \includegraphics[width=\textwidth]{histo2.jpg}
     \caption{}
     \label{fig:three sin x}
 \end{subfigure}%<---
 \begin{subfigure}[b]{0.33333\textwidth}
     \centering
     \includegraphics[width=\textwidth]{histo3.jpg}
     \caption{}
     \label{fig:five over x}
 \end{subfigure}
    \caption{ Histogram Plot of (a) Input Image, (b) Encrypted Image and (c) Decrypted Image }
    \label{fig:three graphs}
\end{figure}

\end{document}

Answer 2

This uses \allowbreak to automatically break a line of subfigures without using up space in the process.

\documentclass{article}
\usepackage{graphicx}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{algorithm}
\usepackage{amsmath}

\begin{document}
\begin{figure}
 \centering
 \begin{subfigure}[b]{0.5\textwidth}
     \centering
     \includegraphics[width=\textwidth]{example-image-a}
     \caption{}
     \label{fig:y equals x}
 \end{subfigure}\allowbreak
 \hfill
 \begin{subfigure}[b]{0.5\textwidth}
     \centering
     \includegraphics[width=\textwidth]{example-image-b}
     \caption{}
     \label{fig:three sin x}
 \end{subfigure}\allowbreak
 \hfill
 \begin{subfigure}[b]{0.5\textwidth}
     \centering
     \includegraphics[width=\textwidth]{example-image-c}
     \caption{}
     \label{fig:five over x}
 \end{subfigure}% note, \caption starts and ends with a \par
    \caption{ Histogram Plot of (a) Input Image, (b) Encrypted Image and (c) Decrypted Image }
    \label{fig:three graphs}
\end{figure}
\end{document}