\documentclass{article}
\usepackage{bussproofs}
\begin{document}
\begin{prooftree}
\AxiomC{$P$}
\AxiomC{$\neg P,Q$}
\RightLabel{\scriptsize Res}
\BinaryInfC{$Q$}
\AxiomC{$\neg Q,R$}
\RightLabel{\scriptsize Res}
\BinaryInfC{$R$}
\end{prooftree}
\end{document}