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.

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

With Mars in your third house you’re driven to pursue knowledge, and are very motivated when it comes to all forms of communication. With your great mental energy you'd make a vital and stimulating teacher. If this energy is not disciplined and controlled, however, it will accentuate your propensity towards being high-strung, nervous, and quarrelsome with siblings and neighbors. Meditation, regular exercise, or therapies are some ways to master this predisposition. You're often impulsive in your thinking, and need to exercise proper caution and judgment before acting. You will have many short journeys, particularly in your adolescence. If Mars is afflicted, you are prone to accidents in those short trips. Often a brother or a sister will be in the armed services, or perhaps connected to law enforcement in some way. If your Mars is heavily afflicted, you may experience the sudden death of a relative through an accident.

If Mars is within 10° of the nadir (the fourth house cusp), then it is conjunct the fourth house cusp and much of the foregoing interpretation starts to become weaker. If Mars is in the third house AND within 4° or 5° of the nadir, then it is said to be on the "dark side" of the fourth house, and it influence will be felt in the fourth as well as in the third house. The closer a third house Mars is to the nadir, the more it will be interpreted as though it were in the fourth house; when less than 3° from the nadir, a third house Mars expresses itself primarily—and eventually entirely—in the fourth house.