How to find this house
Find the ascendant sign and count it as house one. Continue one sign at a time until house 3. The entire sign is assigned to that house in the whole-sign method used by the Kalika beta.
A house can be empty of planets. Its existence and sign do not depend on whether a planet appears inside it.
Traditional study topic
The third house follows the second sign. Traditional study often associates it with initiative, siblings, and communication.
These associations organize a tradition of interpretation. They are not outcomes calculated from a house number, and different schools may give topics different emphasis.