Mercury in the 3rd house (Sahaja, the house of courage) brings the significations of Mercury, namely intellect & analysis, communication & speech, business & trade, into the area of courage, self-effort, siblings and communication. The 3rd house governs courage & effort, younger siblings, communication & skills, short journeys.
Results of Mercury in the 3rd House
Mercury is a neutral planet, so here it tends to bring poor communication, indecision, nervousness and trouble in studies or trade focused on courage, self-effort, siblings and communication.The exact result depends on which sign Mercury occupies here and the aspects it receives.
When It Activates
These results typically surface during the Vimshottari mahadasha or antardasha of Mercury, and when transiting planets trigger the 3rd house. A well-placed Mercury here supports bravery, initiative, talent and good bonds with siblings; when afflicted it can indicate timidity, lack of drive and strained sibling relationships.