There are multiple ways to determine how you would describe yourself in three words. First you can describe yourself by the things you love to do. Another way is to ask your closest friends to describe you. Finally you can describe yourself as the person you're aspiring to become.
According to LifeHack.org, a helpful tip when it comes to describing yourself is to create a sentence that starts with, "I am someone who..." Doing this eases the pressure of selecting three specific adjectives to describe yourself. Performing this tactic could also help you achieve three words. Leading with "I am someone who..." puts you in a position where you can talk about yourself freely.