In traditional astrology, the third house represents your style of communication, day to day thinking, intelligence, primary education, determination, perceptions, memory, short distant travels, and speech. It also shows what your relationship to your siblings and neighbors is bound to be like unless consciously transformed.
Supplementing traditional astrology, the esoteric representation of the third house is the integration of the higher and lower self.
Gemini is the original ruler of the third house and Mercury is the traditional ruler. Once an individual is dedicated to their spiritual path (esoteric astrology), Venus becomes the ruler of the third house.
Depending on your natal ascendant and planetary placements, your third house signs and planets could be different from the original.