I had septorhinoplasty/turbinate reduction 3 wks ago. I now sleep on my back; can I wear an eye mask to block ambient light?

I don't imagine that a thin lightweight eye mask would apply enough weight or pressure to alter your nose three weeks after surgery. That said, your surgeon is the only one that knows the specific details of your operation and whether a sleep mask poses any undue risk. In these situations, it is always best to check with your surgeon. All my best.

I would ask your plastic surgeon because each physician has a different protocol and it depends on whether or not the nose was fractured.  I usually advise patient to avoid any heavy pressure on the bridge of the nose from glasses or things such as masks and goggles for the first 3-4 weeks after surgery. 

Typically, for my patients that have undergone septorhinoplasty, a light cloth or eye mask would not be an issue at 3 weeks after surgery.