Si tu caso es para 3 decimales sería así:
DATA: ltd_lvc_t_fcat TYPE lvc_s_fcat.
DATA: fila TYPE i VALUE 1,
columna TYPE i VALUE 1,
size TYPE char3.
size = 16.
CLEAR ltd_lvc_t_fcat.
ltd_lvc_t_fcat-row_pos = fila.
ltd_lvc_t_fcat-col_pos = columna.
ltd_lvc_t_fcat-tabname = 'GIT_ALV'.
ltd_lvc_t_fcat-fieldname = 'IMPORTE'.
ltd_lvc_t_fcat-just = 'R'.
ltd_lvc_t_fcat-scrtext_l = text-t01.
ltd_lvc_t_fcat-fix_column = 'X'.
ltd_lvc_t_fcat-decimals = '3'.
ltd_lvc_t_fcat-outputlen = 13.
APPEND ltd_lvc_t_fcat TO g_t_fieldcat.
Saludos
