OK Crescencio
I have the comboboxes inside the grid.

For each combobox:

The first one
rowsourcetype=6 (Fields)
rowsource=tipimp.codimp,desimp

The second one
rowsourcetype=6 (Fields)
rowsource=aimtabd1.aimtcod1,aimtdes1

The third one
rowsourcetype=6 (Fields)
rowsource=aimtabd2.aimptas3