Yes. For the most part, it is safe. However, there are certain things to be mindful of. If your doctor has told you not to have intercourse because of the position of your placenta, ie, placenta previa, then it is not safe. The other thing to be careful about is that sex can stir up contractions, so even though you may be in your third trimester, it can sometimes cause you to go into preterm labor.
Yes. Use protection nothing too forceful.