PARAMETERS: fname_1(100) modif id LCK.
PARAMETERS: fname_2(100) modif id LCK.
PARAMETERS: fname_3(100) modif id LCK.

DATA: fname(1024).

DATA: gt_fname type FILETABLE with header line.
data: rc type i.

at selection-screen output.
  loop at screen.
    if screen-group1 = 'LCK'.
      screen-input = 0.
      modify screen.
    endif.
  endloop.

at selection-screen on value-request for fname_1.

  CALL METHOD cl_gui_frontend_services=>file_open_dialog
    CHANGING
      file_table              = gt_fname[]
      rc                      = rc
  .
  read table gt_fname index 1.
  if sy-subrc = 0.
    fname = gt_fname.
    fname_1 = fname(100).
    fname_2 = fname+100(100).
    fname_3 = fname+200(100).
  endif.

  data: dynpfields type table of DYNPREAD with header line.

  dynpfields-FIELDNAME = 'FNAME_1'.
  dynpfields-FIELDVALUE = FNAME_1.
  append dynpfields.

  dynpfields-FIELDNAME = 'FNAME_2'.
  dynpfields-FIELDVALUE = FNAME_2.
  append dynpfields.

  dynpfields-FIELDNAME = 'FNAME_3'.
  dynpfields-FIELDVALUE = FNAME_3.
  append dynpfields.

  CALL FUNCTION 'DYNP_VALUES_UPDATE'
    EXPORTING
      dyname                     = sy-cprog
      dynumb                     = sy-dynnr
    tables
      dynpfields                 = dynpfields.

START-OF-SELECTION.
  write fname.