Hairline crack Apple watch

see "Don’t use a damaged Apple Watch, such as one with a cracked screen or case, visible liquid intrusion, or a damaged band, as it may cause injury."

in the article

Important safety information for Apple Watch – Apple Support

Are you sure that's actually a crack? From the photo, it looks like a scratch. I'd schedule an appointment at your Apple Store Genius Bar and have the watched checked out. If it is cracked, I wouldn't invest in having it repaired at this point. Apple Watch Series 3 is not very out of date and for the investment in repair, it would make more sense to apply the cost you'd make in the watch towards a new watch.