Is it safe to lay on my back during the third trimester of pregnanct?

Yes, but...: The uterus can rotate to the right & impact blood flow back to the heart from compression of the vena cava. Probably the best position for sleep in the 3rd trimester is on the left side in fetal position. That maximizes blood flow to the placenta which is how the baby gets nourishment.

Slight tilt better: There's a large vein (vena cava) that returns blood to your heart that runs up on the right side near your spine. It's possible to compress this with your uterus & that's why you may feel light-headed, dizzy, or with palpitations when you lay on your back. Simply a slight tilt towards your left can avoid vena-caval compression & you'll feel better. If no symptoms, i wouldn't worry about it.