The Sun in the 3rd is classically excellent: courage, initiative and success through one's own effort. The person is bold in communication and often supports or leads siblings.
What the 3rd house governs
The 3rd house, Sahaja bhava, covers courage, effort, younger siblings, communication, short journeys and skills. It is an upachaya (growth) house, where results improve with time and effort.
What Sun brings
Surya, the Sun, is the king among the grahas and the natural soul-significator (Atmakaraka). It represents vitality, authority, the father and one's sense of self.
What changes the result
- Sign and dignity: Sun exalted or in its own sign gives the best version of this placement; debilitated or in an enemy sign, the weaker version.
- House lordship: which houses Sun rules for your Lagna decides whose matters it carries into the 3rd house.
- Aspects and conjunctions: Jupiter's aspect protects; Saturn, Mars or Rahu add pressure.
- Dasha: the placement shows most strongly during Sun's own Mahadasha or Antardasha (6-year Mahadasha).