Help in ALV Grid

Hi All,

I am having 3 field in alv grid say fld1,fld2 and fld3.

fld1 and fld2 is editable. I want to put some value in fld1 and

fld2 and i want the differance should come in fld3.

how can I do. pls give some logic.

I wrote in user comman but it is not reflected.

FORM user_command USING r_ucomm LIKE sy-ucomm

rs_selfield TYPE slis_selfield.

ENDFORM.