What are the consequences of intentionally breaking a day of make-up fasting without a valid excuse, and is it obligatory to fast three days thereafter?

Fasting to make up for missed Ramadan days is obligatory, and it is not permissible to break it except for a legitimate Shar'i excuse. Whoever breaks their fast during make-up Ramadan fasting must make up for that day. If the breaking of the fast was without a valid excuse, then repentance is also required. There is no basis for fasting three days instead of one, and the correct view is that only one day needs to be made up, because it is not permissible to add more than one day of make-up fasting without a text or scholarly consensus.