Table width equal line width

I would suggest stacking the headings, and perhaps eliminate the duplication of "Number", since it should be obvious:

\documentclass[twocolumn]{article}
\usepackage{booktabs,lipsum}% http://ctan.org/pkg/{booktabs,lipsum}
\begin{document}
\lipsum[1]

\begin{table}
  \centering
    \begin{tabular}{ccc}
        \toprule
        Generated & Real & Ratio of \\
        cluster & cluster & mapped spikes\\
        \midrule
        2 & 3 & 0.4233 \\
        3 & 2 & 0.3545 \\
        4 & 1 & 0.8463 \\
        5 & 1 & 0.3432 \\
        6 & 3 & 0.6345 \\
        7 & 2 & 0.2349 \\
        8 & 2 & 0.4267 \\
        \bottomrule
    \end{tabular}
  \caption{A table}
\end{table}

\lipsum[2-5]
\end{document}

I've used booktabs, which inherently discourages the use of vertical lines. However, the example would work even without if you reinstate the vertical and horizontal rules. booktabs just provides the necessary eye-candy for tabular.

You can use the starred version of tabular environment and a p{} modifier for column alignment with custom size, based on \linewidth.

\documentclass[twocolumn]{article}
\usepackage{booktabs}
\usepackage{lipsum}
\begin{document}
\begin{table}[t]
\centering\small
\begin{tabular*}{\linewidth}{@{\extracolsep{\fill}}p{0.3\linewidth}p{0.3\linewidth}p{0.3\linewidth}@{}}
\toprule
Generated Cluster Number & Real Cluster Number & Ratio of mapped spikes \\
\midrule
         2 &          3 &     0.4233 \\
         3 &          2 &     0.3545 \\
         4 &          1 &     0.8463 \\
         5 &          1 &     0.3432 \\
         6 &          3 &     0.6345 \\
         7 &          2 &     0.2349 \\
         8 &          2 &     0.4267 \\
\bottomrule
\end{tabular*}
\label{tab:template}
\end{table}
\lipsum[1-5]
\end{document}