The Third House is related to thoughts and to communications. Misunderstandings and accidents are also covered by this, as are routine interactions and journeys - for example, a journey to work. In addition, it has a connection with buying and selling and with our relationship to our neighbours and our brothers and sisters.
Having many planets in this house indicates a large number of interactions with other people. The energies of some planets clash with that of the Third House: for example, Neptune is probably too holistic in its nature. A person with Neptune in the Third House might have a way of thinking which is too vague and woolly to be really useful.