colortbl: \rowcolor in tables with \begin{tabular}{@{}ccc@{}}

\documentclass[a4paper,12pt]{article}
\usepackage{colortbl}
\newcolumntype{C}{c<{\kern\tabcolsep}@{}}
\begin{document}

\begin{tabular}{@{}CCc@{}}\hline 
  \rowcolor[gray]{.9}[0pt][0pt] a & b & c \\\hline
\end{tabular}
\end{document}

From the userguide of the colortbl package (which provides the commands \columncolor and \rowcolor):

\rowcolor takes the same argument forms as \columncolor. It must be used at the start of a row. If the optional overhang arguments are not used the overhangs will default to ... \tabcolsep ... [emphasis added]

Here, the length \tabcolsep is (half) the width of the intercolumn white space, and equals 6pt by default. However, specifying the \rowcolor command as

\rowcolor[gray]{.9}[][] % empty contents resolve to 0[pt]

will not produce what you're after either, I'm guessing, because this will leave white gaps in the table's two interior columns. The colortbl package doesprovides a \cellcolor command, but unfortunately this command doesn't take left- or right-hand trimming options, so you'd be back where you started out with.

In short, if you insist on using the \rowcolor command, you're probably best served by omitting the @{} specifiers at the far left and far right of the tabular specification.