Astronomically speaking, the third house is that one-12th sector of the local sky (see above picture) clockwise from the nadir occupied by planets in the two hours after they pass through the lowest point of the local sky.

The third house represents understanding through information, and what we know as a result of interactions within our community. "Community" here means neighbors, siblings, short journeys, letters, news and rumors. This house rules our rational thought and conscious mind, memory, speech, early education and environment, writing and communication, dexterity and taken-for-granted skills, explorations, investigations, inquiry, and research of all kinds. Planets in the third house describe the quality and quantity of our mental and nervous energy.

This Third House of Astrology Houses page and much of this 550-page resource website are taken from You and the Universe.