TikZで集合の図を描く方法

TikZで集合の図を描く．

\documentclass[border=15pt]{standalone}
\usepackage[]{tikz}
\usetikzlibrary{backgrounds,fit} 
\begin{document}
   \begin{tikzpicture}[scale=3]
       %node
       \node[above] (a_1) {$a_1$};
       \node[below right] (a_2) {$a_2$};
       \node[below left] (a_3) {$a_3$};

       %枠
        \begin{scope}[on background layer]
            \node[
                label={
                    [label distance=-5pt,fill=white]90:$A$
                    %label distanceを短くするのがポイント
                    },
                draw,circle,
                fit=(a_1) (a_2) (a_3)
                ] {};
        \end{scope}
   \end{tikzpicture} 
\end{document}