假设B.COL3是字符串，则：

SELECT A.COL1,
       A.COL2,
       CASE
       WHEN COUNT(B.COL3) > 1
       THEN 'More than one Value'
       ELSE MAX(B.COL3)
       END AS COL3
FROM   TBL1 A
       RIGHT JOIN TBL2 B
       ON A.TBL1_ID = B.TBL2_ID
GROUP BY A.COL1, A.COL2;

如果不是字符串，则使用TO_CHAR(MAX(B.col3))。
其中，对于示例数据：

CREATE TABLE tbl1 (col1, col2, tbl1_id) AS
SELECT 'A',  'B',  1 FROM DUAL UNION ALL
SELECT 'A1', 'B1', 2 FROM DUAL;

CREATE TABLE tbl2 (col3, tbl2_id) AS
SELECT 'C',  1 FROM DUAL UNION ALL
SELECT 'C1', 2 FROM DUAL UNION ALL
SELECT 'C2', 2 FROM DUAL;

输出：
| COL1| COL2| COL3|
| - ------|- ------|- ------|
| A类|B| C级|
| A1|地下一层|多个值|
fiddle