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.

Your thoughts and ideas are deep, intense, and under a lot of pressure to change, evolve and grow. Life is forcing you to leave your prejudices and incorrect ways of thinking behind. If your Pluto is unafflicted, this is a wonderful placement for a constant regeneration of your thoughts, as well as a source of inspiration and new insights. In this case you'd make a good psychologist, detective, or someone who needs to look deeply below the surface of things. If your Pluto is afflicted here, however, you may be prone to compulsive or obsessive thinking, or try to control the thinking of others. If Pluto is also in hard aspect to Mercury or Neptune, there can be more serious mental problems.

If Pluto 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 Pluto 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 Pluto 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 Pluto expresses itself primarily—and eventually entirely—in the fourth house.