DB1=
  (
  (ADDRESS = (PROTOCOL = TCP))
   ...
   ...
  (CONNECT_DATA = (SID = db1))
  )

DB2=
  (
  (ADDRESS = (PROTOCOL = TCP))
   ...
   ...
  (CONNECT_DATA = (SID = db2))
  )

DB3=
  (
  (ADDRESS = (PROTOCOL = TCP))
   ...
   ...
  (CONNECT_DATA = (SID = db3))
  )

DB_ONE=
  (
  (ADDRESS = (PROTOCOL = TCP))
   ...
   ...
  (CONNECT_DATA = (SID = db1))
  )

DB_TWO=
  (
  (ADDRESS = (PROTOCOL = TCP))
   ...
   ...
  (CONNECT_DATA = (SID = db2 ))
  )