Hello Jeanette,
If your scenario is like
PO->Delivery->TO
then following tables would help you.
Get delivery(vbelv/posnv) from VBFA by passing TO number and item number in vbeln/posnn with vbtyp_n = 'Q'
Get PO number(vgbel/vgpos) from LIPS by passing above delivery in vbeln/posnr.
Get PO details from EKPO by passing above PO number in ebeln/ebelp.
Hope this helps you.