The 3rd house in astrology is a cadent house meaning it is about change and adaptation. Cadent housed helps us evolve. The 3rd house is about how we adapt to social changes, given this is an air house, in our immediate environment. The native location for Gemini, the element of this house is air, the ruler of this house is Mercury. This house is where you can find information on childhood, neighbors, peers, and siblings, how you communicated with others in every form of communication, and how you think. The third house shapes the conscious mind and quick reasoning. Planets here take on many 3rd house qualities if there are enough planets here the person could seem like a full-blown Gemini.
If you are looking at someone’s natal chart are judging the conditions of transits in relation to a natal chart each planet especially the personal planets: Sun, Moon, Mercury, Venus, and Mars will have the same feel as a Mercury conjunction. For the Sun that means someone who loves to communicate and make their point known. They can argue up a storm if it comes down to it. For the Moon, its placement here would create a need to talk or communicate in all forms to sort out the things going on within. Friends and peers can come and go like the tide. The inner day to day mind and emotions are riddled with mercurian nervous energy. With Mercury here, you will have one cunning and smart individual on your hands. They are smart because of an insatiable curiosity. Having their hands in more than one cookie jar is how they keep some semblance of inner peace. It takes time for the Mercury in the third to develop the staying power to handle multiple things at once. Venus here makes a flirt, they will flirt with their friends, neighbors, the postman, you name it. They value communication in all forms. It is like an art for them. In fact, if this person is creative or artistic, they will use creativity to communicate what they value. Their words are subtle used they will use sweet words to get what they want. Mars here makes a spitfire. I do not wish to battle with someone who has this Mars placement. Just with how they communicate form body language to words or text they make it known they can tear you a new one.
The third house sign in someone natal chart tells you the basic of how someone communicates, and people they deal with on a daily bases. To look more in-depth you will have to find the lord of the third house, meaning you would have to find the planet that rules the sign of that person’s third house.