Try this:
Code:
SQL> COL col3s format a60
SQL> SELECT     col1, SUBSTR (MAX (SYS_CONNECT_BY_PATH (col3, ',')), 2) col3s
  2        FROM (SELECT col1, col3,
  3                     ROW_NUMBER () OVER (PARTITION BY col1 ORDER BY col3) rn
  4                FROM temp1)
  5  START WITH rn = 1
  6  CONNECT BY PRIOR rn = rn - 1 AND PRIOR col1 = col1
  7    GROUP BY col1
  8  /

COL1           COL3S
-------------- ------------------------------------------------------------
1              Val1,Val2,Val3,Val4,Val5
2              Val6,Val7,Val8
3              Val10,Val11,Val12,Val9