To answer your first question, a three-way match compares the voucher you are entering to:
• The original purchase order
• The supplier's invoice
• The purchase order receipt record for the items that were received (like a G/L Post Report)
Three-way matches are generally used for purchases of finished goods. The item must have a purchase order receipt record showing that it was received before you can enter a voucher for it using three-way match. This helps you avoid paying for items that you have not received, but for which the supplier has billed you.
I hope that answers your question.