The Third House is the house of communication: speaking, writing, learning, and the exchange of information. It is the house of siblings, neighbors, short journeys, and the immediate environment — the world you encounter on a daily basis, close to home.
In traditional astrology, the Third House is called the House of Brothers (or Sisters). It describes your relationships with siblings, your experience of early education, and the quality of your everyday communication with the people around you.
A planet in the Third House describes how you communicate and learn. Mercury in the Third House is at home, giving a quick, curious mind and a gift for language. Mars in the Third House gives a direct, confrontational style of communication, and often conflict with siblings or neighbors. The Moon in the Third House gives an emotional quality to communication, a tendency to speak from feeling rather than analysis.
The Third House is naturally associated with Gemini and Mercury: the sign of communication and the planet of thought. When planets are placed in the Third House, they take on something of this Geminian quality, becoming more communicative, more curious, more focused on ideas and information.