Third House in astrology also known as 'Prakarma Bhav' or 'Sahaj Bhava' ie. House of Courage and Valour. This house mainly signifies a person's ability to fight back and strength to face the atrocities of life. It shows how hardworking the person is.
It tells us how much perseverance a person can have in the difficult times and how committed one is in performing the duties and responsibilities. 

Being next to 2nd house i.e. House of Parents, this house relates to the immediate environment we form after parents and that is our siblings especially younger siblings and one's relationship with them. This relationship is at both the conscious and subconscious level. It also tells about the relation with immediate neighbours and the surrounding environment.

Another major aspect that this house governs is all sort of communication and its modes such as Media, TV, Radio, Internet, Telephone, Social media, Newspaper, Telegraphy, etc. Anything related to exchange of information whether verbal or written is dealt from this house.

Short Distance travels and its success is majorly seen from this house. Therefore any profession related to travel and communication is being seen through this house. Various examples of such are Tourism, Transportation-Railways, Airways, Roadways, Waterways, Media, Broadcasting, Information technology, Journalism, Writers, Publishing, etc.

The third house also deals with one's intellect, aptitude or mental intelligence of the person. It tells us about one's interests and bent of mind concerning hobbies and what a person wants to pursue. This is why it also deals with the quality and type of early education one receives in the primary years of life. It signifies how well the person can grasp the knowledge in his surrounding environment through different modes of communication and then use it as their means of self-expression. 

The third in the zodiacal series is Gemini and hence the natural ruler of this house is Mercury which governs intellect, communication and travel. Hence this house is mainly all about this.
The upper chest, shoulders, thymus gland, trachea, lungs, collar bone, right ear, right arm and hand, etc are the body parts ruled by the third house.