Sleeping sideways and crib sleeping

My baby is 3 months old and sleeps inside the rocker right next to me. I keep catching my baby turning side ways. Either trying to sleep like that or is already side ways sleeping. I have to always put my baby back on his back. I want to put my baby into his crib now. If you were in my situation or was in a similar situation, how did you guys manage to put your baby inside the crib and feel comfortable leaving your baby there knowing your baby flips side ways and try’s to sleep like that? My baby’s crib is inside my room. I just feel scared to put my baby inside the crib knowing he’s doing that already inside his rocker. The only reason why i’am a bit clam now it’s because his rocker is right next to my bed. Also how does that work when your baby sleeps wearing a sleeping sack?