Where do I get the 3 digit security code for my Apple Pay, not Apple Cash but the account you can add money to from an apple gift card?

I’m trying to make a purchase using my Apple Pay that has money added from a gift card, but it asks me to enter the 3 digit security code on the back of the card. I never got a card for Apple Pay so I don’t know what security code to enter. The purchase I’m trying to make is a bus ticket for my son to a rehab facility upstate. Please someone help, I need to have the bus ticket by tomorrow or he won’t get in until another bed becomes available and God knows when that will be. His children need him and I don’t know what else to do because that is the only money I have. I have nothing else. I have nothing left.