If the PO was entered under an order type of "B" for Blanket, you would have released a line and chosen the quantity to release. If the release was for a quantity of 10 and you received only 3, you could go into the release and either close the release (you should wait until it's vouchered) or change the quantity on the line item to 0. Progression will then ask you if you want to cancel the line.
Kevin Scheeler