“Every person has three names, that given to him by his parents, that which others call him, and that which he calls himself.” Midrash
This midrash is pointing out that people very often act differently in different situations. When they are at work, or with friends, parents, teachers or alone by themselves, they may conduct themselves differently and show different sides of themselves.
Our great Torah personalities, however, achieved their potential by being one person to G-d, to others, and to themselves.
We are born with a number of personality traits. These can be channeled in good or bad directions. Surprisingly our sages said that Moshe Rabbenu was born with traits that could have been very obnoxious, but that he transformed them into excellent personality characteristics. Moshe had nothing to hide from others, and he had no need to defend his own character. It had become pure and noble. The self that he knew he had was the one he exhibited to others.
Moshe is referred to as Rabbenu, our teacher, his instruction was not only conveying the word of G-d to the world, but the way in which he lived and transformed his own life, to have one character and blend all three names into one.