\begin{table}%[!ht]%T3
%\centering
\par
\caption{Boundaries between the shells. For each shell nr., the lower
and upper radius (in arcmin) are listed.}
\label{tab:boundaries}
\begin{tabular}{|rc|rc|rc|}
\hline\hline
nr. & $r_1$ -- $r_2$ (\arcmin) &
nr. & $r_1$ -- $r_2$ (\arcmin) &
nr. & $r_1$ -- $r_2$ (\arcmin) \\
\hline
1 & 0--0.5 & 4 & 2--3 & 7 & 6--9 \\
2 & 0.5--1 & 5 & 3--4 & 8 & 9--12 \\
3 & 1--2   & 6 & 4--6 & 9 & 12--16 \\
\hline
\end{tabular}
\end{table}