DECLARE @t1 TABLE(id     INT, 
                  field1 VARCHAR(20), 
                  field2 VARCHAR(20), 
                  field3 VARCHAR(20));
DECLARE @t2 TABLE(id   INT, 
                  cust VARCHAR(20));
INSERT INTO @t1 VALUES (1, 'A', 'B', 'C'), (2, 'D', 'B', 'A'), (3, 'F', 'A', 'B');
INSERT INTO @t2 VALUES (1, 'F'), (2, 'D'), (3, 'D'), (4, 'D'), (5, 'D');

SELECT a.*, 
       ISNULL(b.x, ' ') AS x
  FROM @t1 AS a
       OUTER APPLY
       (
        SELECT TOP 1 'X' AS x
          FROM @t2 AS b
         WHERE b.cust IN (a.field1, a.field2, a.field3)
       ) AS b;


id          field1               field2               field3               x
----------- -------------------- -------------------- -------------------- ----
1           A                    B                    C                     
2           D                    B                    A                    X
3           F                    A                    B                    X