Draw node below two others nodes from the first node west to the second node east

Question

I have this MWE :

\documentclass[tikz]{standalone}
\usepackage[utf8]{inputenc}
\usetikzlibrary{shapes.geometric,arrows,positioning,fit,calc,backgrounds}
\begin{tikzpicture}
\node[draw, rectangle] (r1) {R1};
\node[draw, rectangle, right= of r1] (r2) {R2};
\node[draw, rectangle, below= of r1] (r3) {R3};
\end{tikzpicture}
\end{document}

I want to have this relation for the x coordinate :

r3.east.x = r2.east.x; r3.west.x = r1.west.x

How can I do that? My idea is to position r3 at south of (r1.west + r2.east)/2 and then calculate the right width.