\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{calc}
\setlength{\parindent}{0pt}
\begin{document}
\begin{tikzpicture}
\tikzstyle{branche} = [  ]
\tikzstyle{noeud} = [  ]
\tikzstyle{proba} = [ midway,fill=white,scale=.5 ]
\node[noeud] (N3-0) at ({2*3},0) {$S$};
\node[noeud] (N3-1) at ({2*3},-0.5) {$\overline{S}$};
\node[noeud] (N3-2) at ({2*3},-1.5) {$S$};
\node[noeud] (N3-3) at ({2*3},-2.0) {$\overline{S}$};
\node[noeud] (N3-4) at ({2*3},-3.0) {$S$};
\node[noeud] (N3-5) at ({2*3},-3.5) {$\overline{S}$};
\node[noeud] (N3-6) at ({2*3},-4.5) {$S$};
\node[noeud] (N3-7) at ({2*3},-5.0) {$\overline{S}$};
\node[noeud] (N2-0) at ($0.5*(N3-0)+0.5*(N3-1)+(-2,0)$) {$S$};
\node[noeud] (N2-1) at ($0.5*(N3-2)+0.5*(N3-3)+(-2,0)$) {$\overline{S}$};
\node[noeud] (N2-2) at ($0.5*(N3-4)+0.5*(N3-5)+(-2,0)$) {$S$};
\node[noeud] (N2-3) at ($0.5*(N3-6)+0.5*(N3-7)+(-2,0)$) {$\overline{S}$};
\node[noeud] (N1-0) at ($0.5*(N2-0)+0.5*(N2-1)+(-2,0)$) {$S$};
\node[noeud] (N1-1) at ($0.5*(N2-2)+0.5*(N2-3)+(-2,0)$) {$\overline{S}$};
\node[left,noeud] at ($0.5*(N1-0)+0.5*(N1-1)+(-2,0)$) {$\Omega$};
\draw[branche] ($0.5*(N1-0)+0.5*(N1-1)+(-2,0)$) -- (N1-0) node[proba] {3/7};
\draw[branche] ($0.5*(N1-0)+0.5*(N1-1)+(-2,0)$) -- (N1-1) node[proba] {4/7};
\draw[branche] (N1-0.east) -- (N2-0) node[proba] {3/7};
\draw[branche] (N1-0.east) -- (N2-1) node[proba] {4/7};
\draw[branche] (N1-1.east) -- (N2-2) node[proba] {3/7};
\draw[branche] (N1-1.east) -- (N2-3) node[proba] {4/7};
\draw[branche] (N2-0.east) -- (N3-0) node[proba] {3/7};
\draw[branche] (N2-0.east) -- (N3-1) node[proba] {4/7};
\draw[branche] (N2-1.east) -- (N3-2) node[proba] {3/7};
\draw[branche] (N2-1.east) -- (N3-3) node[proba] {4/7};
\draw[branche] (N2-2.east) -- (N3-4) node[proba] {3/7};
\draw[branche] (N2-2.east) -- (N3-5) node[proba] {4/7};
\draw[branche] (N2-3.east) -- (N3-6) node[proba] {3/7};
\draw[branche] (N2-3.east) -- (N3-7) node[proba] {4/7};
\end{tikzpicture}
\end{document}