Can you explain little more about this sentence "now item no. 03 has been supplied and deleted from the PO subsequently." Does it mean there are any GR posted and cancel for 3rd line item ?
If you've done the GR and then cancel the GR and set deletion flag for this line item, then your system is behaving as per designed.
If you haven't done any GR or any subsequent document, then system will behave like below:
It will be distributed as 6 USD for each two line items. I've tested the same. It is distributing the amount for two line items when I delete the 3rd line item.
Check the Group condition tick for the condition type FRB1.