How to say no to living with his parents after we already said yes?

Changing your mind three weeks before the wedding will cause a massive family crisis and disappointment, but living somewhere you already resent is worse. You have to speak to him tonight, not tomorrow. Tell him the truth, which is that seeing the cupboard set up made the reality hit you in a way you did not anticipate during the engagement, and you know you cannot survive the joint family arrangement without losing your mind and your marriage. You need to propose a trial period or a separate living arrangement immediately, even if it means renting a small flat nearby so he can still see his parents every single day. He will likely react with shock and anger because everyone already said yes and logistics are fixed, so you must absorb that fallout without backing down. You cannot trade a difficult conversation now for years of quiet misery just to keep wedding guests and relatives happy.