you can sleep however you like as long as it os comfortable for you. you may read that left side is better and to avoid sleeping on your back but my gynae said left right back is okay as long as you feel okay. problem with sleeping on the back is that your baby might be too heavy during 3rd trimester and you will not be comfortable. however, sleeping on your back with elevated pillows is best for heartburn.
Yes, you may sleep on your back but elevated or a pillow to wedge you to the side. As long as not flat on the back.
you can sleep like 45 degree sitting down. but jiayo mummy. 3rd trim is all the sleepless night.