you can try this alternate:
data t_out type table of ty_itab. "assuming itab is of type ty_itab
sort itab by col1 col2 col3.
data: var1 like itab-col1, var2 like itab-col2.
read table itab into wa_itab index 1.
var1 = wa_itab-col1.
var2 = wa_itab-col2.
data val type i.
loop at itab into wa_itab.
if wa_itab-col1 = var1 and wa_itab-col2 = var2.
val = wa_itab-col3 + val.
else.
wa_out-col1 = wa_itab-col1.
wa_out-col2 = wa_itab-col2.
wa_out-col3 = val.
append wa_out to t_out.
clear val.
var1 = wa_itab-col1.
var2 = wa_itab-col2.
val = wa_itab-col3 + val.
endloop.
Rgds