\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}