I reckon you get a 500 THB a day fine for overstaying, so 3 days would be 1500 THB. You should have the option to extend your Visa (or exemption), that would set you back about 1900 THB.
No, that's what they thought and got intro trouble because of it. You have to leave 60 days after you entered Thailand, 90 if you extend it. May 4 is the day the visa expires, then you can't use it to ENTER Thailand anymore
Is there ever any problem re-entering Thailand on a overstay < 90 days? Met a guy who had no idea he was on a 2 month overstay, I told him he wouldn't get banned, but i wasn't 100% sure.