This site is useful:
http://www.pindari.com/rtf3.html
{\rtf1\ansi\deff0
\trowd
\cellx1000
\cellx2000
\cellx3000
\intbl cell 1\cell
\intbl cell 2\cell
\intbl cell 3\cell
\row
}
This will give:
---------------------------
|cell 1 | cell 2 | cell 3 |
---------------------------
A row is delimted with \trowd ... \row
Each cell ends with \cell
\cellx
determines the right side of the corresponding cell in twips