The 3rd house corresponds to an air sign and is therefore a house of kama (desire).
Brothers, courage, adventures, own efforts, life, energy, enthusiasm, initiative, motivation, all desires, voice. Fine arts of music, dance, and drama. Actors, dancers, singers, directors, producers, organizers. Hearing, mental stability, firmness of personality, neighbors, letters, communications, writings, servants, travel (short journeys), hands, arms, shoulders, right ear, breasts, life.
Aside from Its rulership of brothers and sisters, the 3rd house is significant in that it governs desires in general as well as courage, adventures, and one's own efforts. What is revealed is whether one's desires are fulfilled and how much effort is required towards those ends. If the 3 desire houses (3rd, 7th, and 11th) are very strong, a person will get most anything he sincerely desires, even if specific astrological indications say otherwise. For example, if a horoscope contains a heavily afflicted 4th house (by rulership and occupancy), the person may still obtain his own home if he genuinely desires one and the desire houses are very well fortified. It is therefore necessary to analyze the 3rd, 7th, and 11 th houses before giving any final judgement.
The 3rd house as ruler of adventures is also important. It will tell of a stagnant life or one that is filled with constant activity, excitement, and adventures. Also, since it governs courage, this house, when severely afflicted, indicates a person who is fearful. As indicator of brothers and sisters, the 3rd, when afflicted, is said in the ancient Hindu scriptures to produce "no happiness from brothers". This is a good way to make the statement, because there will either be no siblings whatsoever, an antagonistic relationship with siblings, or a sibling who suffers or dies early. It is often impossible to know exactly which of these indications is the case.
The 3rd lord is considered bad in terms of its rulership disposition; therefore the placement of this planet will not much enhance the house it occupies. Generally not causing great harm, it is found to give lukewarm effects.
The karaka or indicator of the 3rd house is Mars.
The retrograde owner of the 3rd house — a karmic relationship with a younger brother (sister), some indecision, fear of risk. In some cases — about talent in writing, preaching, drama.
Source: «Ancient Hindu Astrology for the Modern Western Astrologer» by James Braha