What do the sun, the moon and Venus represent? Are all three constellations the same for the same person?

It's different. The Sun Constellation represents your external performance, the twelve constellations are the Sun Constellations, the Moon Constellation represents your inner heart and relationship with your mother, the Ascending Constellation represents your appearance and temperament, and the Venus Constellation represents your emotional expression and aesthetics. The three constellations of the same person may be the same, but the probability is relatively small, depending on the time and place of your birth.