I'm 17 weeks pregnant with triplets and go to sleep on my sides but often times wake up on my back and my arms are asleep. Is it affecting the babies?

No. Try to sleep on your left side, it will keep pressure off your aorta and provide better blood supply to the babies. All the best and I hope all goes well.
No. You are basically a protective environment for baby.Babies placenta extracts the proper nutrients, and you just carry the fetal-placental unit around. The sometimes concerning symptoms you may have will generally not effect baby. The exception would be changes in your blood pressure, dehydration, significant illness, etc..These things change the "environment" for baby.