2 way matching is when the received invoice is matched against the corresponding Purchase Order, doing this not only validates the quality of the OCR data capture it also checks that you are being billed for what you have ordered.
3 way matching also checks that the invoice and the Purchase Order correspond to what you have received at goods in. It is not uncommon to find wrong products or wrong quantities arriving at goods in. Sometimes this is simply a mistake, and sometimes you may have received a substitute product or part shipment; whatever the circumstance you need visibility of the problem.