Thank you for your answer, after many try I found how I can do it : actually I used “awk” to extract columns on new file like this :
awk '{print $3 ,$2, $7}' $ datafile.txt > gravipoints.txt.
Each $ followed by number matches with a column in original data files after, i used
gmt psxy gravipoints.txt -R -JQ -Sc0.09 -C$CPT -O -K -X -Y >> $PLOT
to get map with points .