3 way matching is to compare three documents i.e.

1. Purchase order

2. Receipt of Items/Packing slip

3. Invoice.

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..

You have to match the invoice with the purchase order and packing slips.

Quantity/Price Ordered should match with Quantity/Price Recieved and Billed is known as Three way match. If the Quantity received and billed is lesser than Qty Ordered is fine to proceed for payment it should not cross Qty Ordered in any circumstance.