WITH t AS
(
SELECT 1 rna, null rnb, null rnc FROM dual
UNION ALL SELECT    2, null, null FROM dual
UNION ALL SELECT null,    1, null FROM dual
UNION ALL SELECT null,    2, null FROM dual
UNION ALL SELECT null,    3, null FROM dual
UNION ALL SELECT null, null,    1 FROM dual
UNION ALL SELECT null, null,    2 FROM dual
UNION ALL SELECT null, null,    3 FROM dual
UNION ALL SELECT null, null,    4 FROM dual
)
SELECT MIN(rna) rna
     , MIN(rnb) rnb
     , MIN(rnc) rnc
  FROM t
 GROUP BY COALESCE(rna, rnb, rnc)
 ORDER BY COALESCE(rna, rnb, rnc)
;