Here is some code to do a full outer join on three tables.

In the example, the three full outer joins are done on two columns , but it works just the same if you only have a single join column.

CREATE TABLE ZA (T CHAR(1), O CHAR(1))
CREATE TABLE ZB (T CHAR(1), O CHAR(1))
CREATE TABLE ZC (T CHAR(1), O CHAR(1))

INSERT INTO ZA (T, O) VALUES ('1', 'A')
INSERT INTO ZA (T, O) VALUES ('2', 'B')
INSERT INTO ZA (T, O) VALUES ('3', 'C')
INSERT INTO ZA (T, O) VALUES ('4', 'D')

INSERT INTO ZB (T, O) VALUES ('1', 'A')
INSERT INTO ZB (T, O) VALUES ('2', 'B')
INSERT INTO ZB (T, O) VALUES ('5', 'E')
INSERT INTO ZB (T, O) VALUES ('6', 'F')

INSERT INTO ZC (T, O) VALUES ('2', 'B')
INSERT INTO ZC (T, O) VALUES ('3', 'C')
INSERT INTO ZC (T, O) VALUES ('4', 'D')
INSERT INTO ZC (T, O) VALUES ('5', 'E')
INSERT INTO ZC (T, O) VALUES ('7', 'G')





SELECT 
	ISNULL(ISNULL(A.T, B.T), C.T),
	ISNULL(ISNULL(A.O, B.O), C.O)
	
FROM
	ZA A FULL OUTER JOIN 
	ZB B ON A.T = B.T AND A.O = B.O FULL OUTER JOIN 
	ZC C ON ISNULL(A.T, B.T) = C.T AND ISNULL(A.O, B.O) = C.O




DROP TABLE ZA
DROP TABLE ZB
DROP TABLE ZC