\documentclass[10pt,a4paper]{book}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{graphicx}
\usepackage{caption}
\usepackage{subcaption}

\usepackage[left=2cm,right=2cm,top=2cm,bottom=2cm]{geometry}

\begin{document}

\begin{figure}
  \centering
		\begin{subfigure}[b]{0.3\textwidth}   
			\centering  \includegraphics[width=4 cm]{figs/Examples and Figures/Mother Wavelets/meyer/Meyer_wavelet1.png}
			\caption{  } \label{fig:2Clusters_ClusterN2_SigmaT}  
		\end{subfigure}
		
		\begin{subfigure}[b]{0.3\textwidth}              
			\centering  \includegraphics[width=4 cm]{figs/Examples and Figures/Mother Wavelets/meyer/Meyer_wavelet2.png}
			\caption{  } \label{fig:2Clusters_ClusterN2_SigmaQ}      
		\end{subfigure}~
		\begin{subfigure}[b]{0.3\textwidth}                  
			\centering  \includegraphics[width=4 cm]{figs/Examples and Figures/Mother Wavelets/meyer/Meyer_wavelet3.png}
			\caption{  } \label{fig:2Clusters_ClusterN2_SigmaP}   
		\end{subfigure}
		 

\caption{ } \label{  }
\end{figure}
\end{document}