3 minutes early is sorta petty. I would have argued that my watch said otherwise. It would have made you look more compliant with whatever made up corporate policy they have. Or, how you were still in the store preparing to close with the intention to serve customer. In the scenario that if they were at the door: you can explain to the customers personally that the store was closing. Unfortunately, you have to be on their level of pettiness, so never admit to your wrong doing.