Girls, tell me how many days it took you to establish breastfeeding?

Girls, tell me how many days it took you to establish breastfeeding? It's been three days and I still have breast milk. The doctor said we need to supplement with formula. He says he lost 290 grams in just three days. But as far as I know, all babies lose weight in the first few days. And so the doctor gave him formula on the third day, and his stomach started churning and gas. Very restless. And before that everything was fine.