\documentclass{article}
\usepackage{tikz}
\usepackage{graphics,graphicx}
\begin{document}
\begin{figure}[!h]
\begin{minipage}[t]{0.31\linewidth}
\centering
\begin{tikzpicture}
\node(A1)at(0,0){$A_1$};
\node(A2)at(1,0){$A_2$};
\node(A3)at(2,0){$A_3$};
\node(A4)at(1.5,1.5){$A_4$};
\draw[-stealth](A2)--(A4);
\draw[-stealth](A3)--(A4);
\end{tikzpicture}
\caption{$G_1=(V,E_1)$}
\label{G1}
\end{minipage}
\begin{minipage}[t]{0.3\linewidth}
\centering
\begin{tikzpicture}
\node(A1)at(0,0){$A_1$};
\node(A2)at(0,1.5){$A_2$};
\node(A3)at(1.5,0){$A_3$};
\node(A4)at(1.5,1.5){$A_4$};
\draw[-stealth](A1)--(A4);
\draw[-stealth](A1)--(A2);
\draw[-stealth](A3)--(A4);
\end{tikzpicture}
\caption{$G_2=(V,E_2)$}
\label{G2}
\end{minipage}
\begin{minipage}{0.35\linewidth}
\centering
\begin{tikzpicture}
\node(A1)at(0,0){$A_1$};
\node(A2)at(0,1.3){$A_2$};
\node(A3)at(1.5,0){$A_3$};
\node(A4)at(1.5,2.5){$A_4$};
\draw[-stealth](A1)--(A4);
\draw[-stealth](A1)--(A2);
\draw[-stealth](A2)--(A4);
\draw[-stealth](A3)--(A4);
\end{tikzpicture}
\caption{$G=(V,E_1\cup E_2)$}
\label{GG}
\end{minipage}
\end{figure}
\end{document}
