PO approval through Position Heirarchy

[naresh.apps11i]

There r three employees say A,B,C in hierarchy.Employee A has raised a PO and he needs to forward to B,C as amount on doc exceeds A's approval amount limit.The requirement is that whenever he forward a doc ,everytime it has to be forwarded to both B& C for Approval and it should get approved by both to show status as "Approved". How this Position hierarchy can be build up in Oracle Purchasing apllication.
Plz explain in detail with Navigation steps.

Earliest reply will be highly appreciated.

Thanks & Regards,
Naresh