Is it ok to let 3 weeks old infant to sleep on tummy? If so, what is the maximum time allowed for infant to sleep on tummy?

My eldest slept on her tummy from newborn till 3 mths old. She slept soundly and better when she was on her tummy but I would turn her over to sleep on her back once I found her sleeping soundly. Sleeping on tummy can be dangerous and need close supervision. Let me share with you my experience. The reason why my gal slept on her tummy was because she couldnt sleep well and always woke up in fright. Hence, my confinement nanny recommended tummy. It went well for the first few days and we began to loosen our guard. Then, there was one day I saw my gal's face facing straight down. She must had moved her head and managed to "suffocate" herself. Her soft cries couldnt be heard coz we were all out in the living room. From that day onwards, she was closely watched as she napped. At night, we did not put her on tummy, only day time.