How to tell my parents we need to call it off when the engagement is in three weeks?

Put the phone down for a second and take a breath. You are staring at a logistical nightmare of advance payments and booked halls, but marrying someone because the tent is rented is a far greater disaster waiting to happen. The longer you wait tonight, the worse tomorrow morning gets. Go straight to your parents right now. Do not try to craft a gentle preamble over a cup of tea or wait for a quiet moment. Sit them down and tell them the truth plainly. You can say that you tried your best to make it work, but the connection simply is not there, and you cannot go through with the wedding three weeks from now. Brace yourself because the next forty-eight hours will be brutal. Your parents will panic about relatives, wasted money, and social embarrassment. Let them be angry and let them process the shock without you snapping back or arguing. They will have to make the hard phone calls to your uncle and the bride's family, and that conversation is going to cause immediate fallout. You have to stand your ground through that storm without folding. Cancelling now causes immense financial and social friction, but it is a temporary pain compared to a lifetime in a mismatched marriage. Every relative will gossip for a week and then move on to the next piece of news. Let them talk. Your only job right now is to stop this train before it reaches the station, take the weight of the family's anger, and protect both yourself and the person you were supposed to marry from a mistake that cannot be undone later.