your PC would have been removed before flight left whatever airport you departed from. Your case will have been scanned and pc removed before flight left.
take it up with the Thai authorities, they are the ones who put the 72 hours window to complete the form. Most of us can count back 72 hours from arriving and fill in the form successfully in that period. Sadly, there are those who constantly argue and want every post altered because they find it difficult.
As I said. 72 hours before you arrive. That is the earliest you can fill it in. That's the start of the count down. Nowhere does it say "before" 72 hours.