Sleeping baby

My sugar is 3 weeks 3 days now and will NOT sleep on her back! I was always so against having her sleep with me, but that is the only time she will sleep/ I get sleep. Her days and nights are mixed up so she barely sleeps at night anyway. She loves to be on her belly on my chest while sleeping & tummy time she falls asleep. I know they say not to lay them down on their belly, but I haven't slept in my bed in 2 weeks and I miss being with my hubby. When can I put her in her crib on her belly to sleep?