Distribution of charges for multiple Purchase order

Dear all,

i have an requirement.

i have 3 vendors in some country. This 3 vendors supply the materials to our customer who is located in another location though an third party vendor.

Once I received the three purchase order materials I need to distribute this fright charges equally to all three PO line items and need to book another invoice for 3 rd party vendor.

This can be done manually. 

But I want this to be done by system. When I select multiple po’s at receive stage.

Let me know how to deal with above process.

Thanks in advance.