I have a triple entry visa, and need to do a re-entry trip. But due to some unforeseen circumstances, I will not be able to make it on time. I'll have to overstay for 3 days. What is the worst cases scenario? Are there any other legal implications besides getting a fine? Does it go on record somewhere and then can be used against me in the future during visa application?

Thank you.

3 days is 1500 baht and a slap on the hand. It will not go against you. Just don't get caught by the police.

Although it does not presently matter for new visa issue it does get recorded both at immigration and in your passport. If you have proof of a flight out a 7 day extension (under review) would be the legal option at 1,900 baht for the TM.7

only 3 days just pay the fine, getting more extension is too much a pain for only 3 days; it won't count against you in any way