How do I draw a box around a venn diagram

I created this venn diagram, and I need to put a box around it as a border, how would I do that?

\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[scale=3, blend group=screen]
  \fill[lightgray] ( 290:.7) circle (1);
  \fill[gray] (150:.6) circle (1);
  \fill[darkgray] (25:.8) circle (1);
\end{tikzpicture}
\end{document}