The third house is the third of the twelve houses of the horoscope. It governs communication, the everyday mind, learning, and short journeys, together with siblings, neighbours, and the immediate surroundings, the near world a person moves through and exchanges with daily. It is a cadent house, the type that precedes the angles; cadent houses are held to be the weakest of the three kinds in force, associated with the mutable, transitional phase of each quadrant and with things in flux and preparation. These are the interpretive conventions of the astrological tradition for this division of the chart, in which the third house describes the connections, messages, and short movements of ordinary life, the mental and local counterpart to the more distant learning and travel of the ninth house across the wheel.
Greek astrologers of the Hellenistic period, whose twelve-place scheme Paulus Alexandrinus set out systematically in 378 after Vettius Valens had already treated it as traditional two centuries earlier, called the third house Thea, the Goddess, and held that the Moon has her joy there, one of seven places across the wheel where a given planet was said to act with particular ease. The pairing fits: the Moon governs the everyday, the near, and the constantly changing, the same territory the third house covers in communication, short journeys, and the press of ordinary local life, so that the fastest-moving body finds its natural home in the house of the nearest and most quickly shifting concerns, the mental equivalent of the Moon's own restless motion through the sky.
Sources A History of Western AstrologyNicholas Campion with Introduction to Astrology (Eisagogika)Paulus Alexandrinus