Oracle 21c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE table_name (colNum, Col1, Col2, Col3) AS
SELECT 1, 123, 341, 635 FROM DUAL UNION ALL
SELECT 2, 323, 434, 343 FROM DUAL;
2 rows affected

SELECT colNum || '-' || col || '-U' AS colNum,
       col123
FROM   table_name
       UNPIVOT(
         col123 FOR col IN (col1 AS 1, col2 AS 2, col3 AS 3)
       )


COLNUM
COL123


1-1-U
123

1-2-U
341

1-3-U
635

2-1-U
323

2-2-U
434

2-3-U
343