3 way matching is to compare three documents i.e.,
1. Purchase order
2. Receipt of Items/Packing slip
For example, if we Order for 50 Items in purchase order and received 40 Items only instead of 50 items. But we received invoice for 50 items. After comparing the three documents we pay for only 40 Items..
For material/Goods Purchases, the Po is required. In the Po, QTY, Product Name, Where is delivery price, Terms and Conditions. In the po there is no matter of final amount. It is just the approximate amount.