One possibility would be to use tabularx. In my MWE i took your table and the package booktabs additionally (and removed the vertical lines and some horizontal ones according to the booktabs manual which resultet in
\documentclass{scrartcl}
\usepackage[fleqn]{amsmath}
\usepackage{booktabs}
\usepackage{tabularx}
\begin{document}
\begin{table}[!ht]
\renewcommand{\arraystretch}{1.5}
\caption[Set of Reports]{ Set of Reports}
\label{Set of Reports}\centering
\begin{tabularx}{\textwidth}{llX}
\toprule
\bfseries Document/Report & \bfseries tbBOSS & \bfseries Remarks \\ \midrule
aaaaaaa & bbbbb & ccccccccccc\\
ddddd & eeeee & ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff fffffffffffffffff\\
gggg & hhhh & iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii \\\bottomrule
\end{tabularx}
\end{table}
\end{document}
Where i further introduced a space in the ffs such that LaTeX can break that (there was no hyphenation available - of course - for ffff... but for your remarks that should work anyways).
which the resulted in the table

The table covers the whole \textwidth according to the first argument of tabularx and while l acts as usual for the first columns, the remaining space is divided equally into the x columns, which is only the last one here.
tabupackage. (otherwise, every question here could be redirected to the TeX book and various manuals, which is not the intention of the site). We welcome you here, and hope that you will enjoy your stay. :) – nickpapior Mar 22 '13 at 08:49