xpointer(/child::FAMILYTREE/child::PERSON[position()=3])
The location path of this XPointer is 
/child::FAMILYTREE/child::PERSON[position()=3].
It is built from two location steps:
/child::FAMILYTREE
child::PERSON[position()=3]
It identifies the single node:
  <PERSON ID="p3" FATHER="p2" MOTHER="p1">
    <NAME>Elodie Bellau</NAME>
    <BORN>11 Feb 1858</BORN>
    <DIED>12 Apr 1898</DIED>
    <SPOUSE IDREF="p4"/>
  </PERSON>