Maybe this would suffice. However, it does not (to my mind) work reasonable spacing in other circumstances (post square root), so I would prefer renaming it as \xsqrt or some unique name to be used sparingly.
RE-REVISED to take Heiko's suggestion on how to remove extra space following the square root:
\documentclass{article}
\usepackage{amsmath}
\usepackage{letltxmacro}
\LetLtxMacro{\oldsqrt}{\sqrt}
\renewcommand{\sqrt}[2][\mkern8mu]{\mkern-8mu\mathop{}\oldsqrt[#1]{#2}}
\begin{document}
$\left(\sqrt{3} \right)$ $\left(\sqrt[3]{4}\right)$
$x\sqrt{3} x\sqrt[3]{3}$
$y=\sqrt{3} = x = \sqrt[3]{2}$
\end{document}

REVISED:
\documentclass{article}
\usepackage{amsmath}
\usepackage{letltxmacro}
\LetLtxMacro{\oldsqrt}{\sqrt}
\renewcommand{\sqrt}[2][\mkern8mu]{\mkern-4mu\mathop{\oldsqrt[#1]{#2}}}
\begin{document}
$\left(\sqrt{3} \right)$ $\left(\sqrt[3]{4}\right)$
$x\sqrt{3} x\sqrt[3]{3}$
$y=\sqrt{3} = x = \sqrt[3]{2}$
\end{document}

\mathop{}\!before the root stuff can be used, then the right side remains unchanged regarding the horizontal spacing. – Heiko Oberdiek Jun 17 '15 at 04:29