The 3rd house is the ground-level house of communication: how you talk, write, and process information day to day, plus the people you cover that ground with most, siblings, neighbors, and early schooling.
It's a cadent house, concerned with processing rather than initiating, and sits opposite the 9th house, where the same appetite for information expands from local exchange into a search for larger meaning.
The 3rd house traditionally rules siblings, neighbors, and short journeys, alongside the everyday mechanics of communication, speech, correspondence, and the flow of local information a person picks up simply by moving through their immediate environment. It also covers early schooling, the first formal setting where a person learns to process and trade information with people who aren't family.
Siblings and neighbors matter here because they're usually the first peers a person deals with, relationships without the built-in hierarchy a parent-child bond carries. That early practice, comparing notes, arguing, trading information as equals, tends to set a default communication style that outlasts childhood by a long way: how directly someone states an opinion, how comfortably they interrupt or get interrupted. Opposite the 9th house, the 3rd house's real character comes into focus: it deals in the local and immediate, what's true here and now, while the 9th house asks whether any of it adds up to something worth believing more permanently.