Three pens for nurses means that nurses in medical institutions have three different colors in their pockets. Nurses are nurses in medical institutions. People in this business have at least three pens in their pockets. One is red, which is used to write DC on the infusion card and draw the body temperature. When the skin test is positive, write smooth paper with blue pen, such as bedside card, blood collection tube, etc. So the nurse's three pens mean that nurses in medical institutions have three different colors in their pockets.