Third House is the house of communication and siblings. Jupiter is expansive and imaginative. Also Third House is ruled by Mars which is a friend of Jupiter. So Jupiter does well in this house. The placement may not be that beneficial but the aspects on Seventh House and Eleventh House definitely make it a dream house to have Jupiter. 

     Jupiter in Third House in a good condition ensures that native has a lot of siblings and all of them are helping and kind natured. Also native gets along well with neighbors and is able to communicate effectively at work. If it is in a bad condition then one can expect the reverse scenario. Third House is the house of creativity and writing. Jupiter is the planet who is positive, imaginative, smart and intuitive. This is a dream combination for Authors and Writers especially those who write friction novels or articulate editorials. Jupiter's aspect on Seventh House blesses both Marriage and Business. If Seventh House is own house or friendly house then one can do great in Business Sector. The other aspect is on Ninth House of Higher Education and Spiritual Enlightment. This aspect also blesses the native to go on long distance travels for Education, Religious or Business Purposes. The greatest gift of aspect on Ninth House is finding the right person or Guru who can elevate the native to a higher conscious. The last aspect is the best of them all and it is on House of Regular Income or Eleventh House. Native is blessed with multiple means of regular income via aspect of Jupiter. Not only that he is also blessed with loving friends and a great social circle. 

     According to B. V. Raman: "Jupiter in third house is in a good position and makes a native optimistic and philosophical. Will have good siblings especially brothers but is a miser. Native may suffer from ill-health. He does not have many friends and cannot use opportunities presented to him". 

     Jupiter is a great placement in third house and aspects are far more beneficial. But for those who have Jupiter along with malefics like Rahu, Ketu or Saturn should pray to Lord Vishnu. If possible one should try to open a business as it may provide favorable results or atleast have a side business to increase source of income. 

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