\psset{treemode=R,nodesep=1mm,levelsep=20mm,treesep=5mm}
%\pstree[treemode=R,levelsep=15ex]{\Tcircle{R} }{
\pstree{\TR{$\Omega$} }{
\pstree{ \Tcircle{$A1$}\taput{$\frac18$} }
    {
    \pstree{ \Tcircle{$A2$}\taput{$\frac18$} }
        {
        \Tcircle{$A3$}\taput{$\frac18$}
        \Tcircle{$\overline{A3}$}\tbput{$\frac78$}
        }
    \pstree{ \Tcircle{$\overline{A2}$}\taput{$\frac78$} }
        {
        \Tcircle{$A3$}\taput{$\frac18$}
        \Tcircle{$\overline{A3}$}\tbput{$\frac78$}
        }
    }
\pstree{ \Tcircle{$\overline{A1}$}\taput{$\frac78$} }
    {
    \pstree{ \Tcircle{$A2$}\taput{$\frac18$} }
        {
        \Tcircle{$A3$}\taput{$\frac18$}
        \Tcircle{$\overline{A3}$}\tbput{$\frac78$}
        }
    \pstree{ \Tcircle{$\overline{A2}$}\taput{$\frac78$} }
        {
        \Tcircle{$A3$}\taput{$\frac18$}
        \Tcircle{$\overline{A3}$}\tbput{$\frac78$}
        }
    }
}