Maimonides Minute – Nonkosher Medicine for Minor Illness – Hilchot Yesodei HaTorah 5:7-8

Although all prohibitions (excepting idolatry, sexual misconduct, and murder) may be violated to save a life, are there times at which they may be violated even when one’s life is not in danger? What is the source for the idea that the three exceptional prohibitions may never be violated even upon pain of death?