Third house is the house of courage and communication. Mercury is the planet which rules speech and communication. Mercury provides mixed results in third house. Mercury makes a native expressive, analytical, artistic and talkative. Such traits can work in their favor sometimes but there maybe times when such traits backfire. 

     According to B. V. Raman: "Natives with Mercury in third house would do good deeds for the benefit of others. Their mind would be sharp and they would be fond of reading and gaining knowledge. They would be gain from business and have good number of siblings. They are liked by friends and relatives for their independent views. They use tact and diplomacy to solve problems at workplace and in general life. If afflicted than they may suffer from nervous breakdown". Mercury is great at communication. But communication isn't just about verbal exchange. In modern day it is about getting your point across in an effective way to clients and consumers. The same rule applies when dealing with friends and families. Those with Mercury in third house have good analytical skills and sharp memory so they are bound to be successful but once in a while their expressive and taunting personality can get them in troubles. 

     Mercury rules the nervous system. Third house controls the nervous system which help relay message from brain to different parts of body. Having an afflicted Mercury in third house can cause some or other sort of nervous disorder or brain tumor. This also holds true if Mercury as third house lord is located in ascendant. There could be nervous breakdown or some disorder which prevents relaying message to other parts of body. In short a native has difficulties in getting the point across if Mercury is in bad condition in the third house. Mercury in third house with Sun gives best results followed by Mars and Jupiter. Mars may give some anger issues and angry speech but gives an excellent career in investigative journalism. Mercury with Saturn in third house may give a thoughtful and calculated approach to life and can make for an excellent mathematician or financial analyst. Mercury with Rahu in third house can give an excellent career in marketing. Mercury with Ketu can make one a journalist. Mercury with Moon can make a good spokesperson which is an asset for any political party. 

     Mercury in third house generally gives good results. For those with career in marketing, sales and communication this placement may indeed be a blessing, There are chances of neurological disorders especially if Mercury is afflicted by bad planets. 

Read our article: Remedies For Mercury to help with weak Mercury.

References:

Raman, Bangalore Venkata. How to Judge a Horoscope. Delhi: Motilal Banarsidass, 2014. Print.