I mistakenly scratched out 3 numbers on my apple gift card

See the following from If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

If you can't read the code on the back of your card because it's scratched or damaged, contact Apple Support.

This is primarily a user-to-user support forum; other Apple users like you.

Please follow the steps in this Apple Support article. 

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

If after reviewing the article, and following the directions in the article, you need additional assistance please follow the link below to contact official Apple Support. 

Contact Apple Gift Card Support

If you receive a request for payment using Apple Gift Cards, or If you believe you are the victim of a scam, please contact Apple Support and report it to local law enforcement.

You may also report the scam to the FTC Complaint Assistant.

Only Apple Support can help with that.