Hi Christina,
If my understanding is correct, your code should be as below:
IF screen-name = 'ZASN-ASNNO'.
SELECT SINGLE ASNNO FROM ZASN INTO LV_ASN WHERE ASNNO = ZASN-ASNNO.
IF SCREEN-NAME = 'ZASN-ASNNO' and LV_ASN+0(3) = 'SCC'.
LOOP AT SCREEN.
IF screen-name = 'ZMT112_DMRRGATE-CONFDA' OR
screen-name = 'ZMT112_DMRRGATE-NTGEW' OR
screen-name = 'ZMT112_DMRRGATE-MAGRV' .
screen-input = 0.
modify screen.
ENDIF.
endloop.
ENDIF.
ENDIF.