1  WITH t AS (SELECT 'A' AS a, 1 AS b, 2 AS c FROM dual UNION ALL
  2             SELECT 'B', 1 , 3 FROM dual UNION ALL
  3             SELECT 'C', 2 , 3 FROM dual UNION ALL
  4             SELECT 'D', 5 , 6 FROM dual UNION ALL
  5             SELECT 'E', 6 , 7 FROM dual UNION ALL
  6             SELECT 'F', 7 , 5 FROM dual UNION ALL
  7             SELECT 'G', 4 , 2 FROM dual UNION ALL
  8             SELECT 'H', 4 , 1 FROM dual UNION ALL
  9             SELECT 'I', 5 , 7 FROM dual)
 10  select t.*,
 11  (select min(a) from t t2
 12  start with t2.a=t.a
 13  connect by NOCYCLE
 14  (t2.b=prior t2.b or t2.c=prior t2.c or t2.b=prior t2.c or t2.c=prior t2.b ) and t2.a <> prior t2.a) o
 15   from t
 16* order by o,a
SQL> /

A          B          C O
- ---------- ---------- -
A          1          2 A
B          1          3 A
C          2          3 A
G          4          2 A
H          4          1 A
D          5          6 D
E          6          7 D
F          7          5 D
I          5          7 D

9 rows selected.