The 3rd house is the house of the immediate environment, contacts, the house of specific thinking, primary and secondary education, short-distance trips that can be covered on foot or by car in one day.
In this article, we are considering the "pure" manifestation of Neptune - that is, if only the planet Neptune is in the third house. Certainly, the presence of other planets can significantly change the picture.
A person with Neptune in the third house perceives most of their knowledge on an intuitive level. They don't really want to strain themselves. As a rule, they will study the subjects that come to them easily and naturally. And most likely, the bias will be towards the humanities, chemistry and biology. And such a person really on some intuitive level feels how to find the right answers. For example, they may write competently, although they do not know all the spelling rules. They just intuitively know how it should be.
The person may have difficulties with clear expression of thought, difficulties with remembering details, with understanding the meaning of what they heard. But such a person can associatively remember information if it hooks them, thereby forming an image, in this way they will better assimilate new knowledge.
There may be difficulty with concentration - those with Neptune in the third house can get stuck, get stuck on something when working with new information. Therefore, such a person may have problems with clear focus, with assimilation of factual information and logical thinking. But they may have a rich imagination and developed figurative thinking and speech, so they have a tendency towards artistic, creative self-expression - poetry, music.
Close environment... They feel the surrounding people well: their mood, their emotional state, and can get involved in this, that is, when communicating with neighbors or friends, they can emotionally empathize with their problems and sympathize with them. But there can also be disappointments and deceptions from this environment, since they idealize their images too much and build false ideas about them.
The same neighbors can be "Neptunian". What does this mean? These are either alcoholics, or drug addicts, or troubles come from them out of nowhere, or they often flood.
Siblings may have oddities, or have confusing relationships with them: they may suffer from psychological disorders, alcoholism (drug addiction), or constantly deceive. Or there may be secrets around them or from the side of relatives.
Trips... On the road, there may be forgetfulness, loss of concentration, some kind of confusion on business trips, on the road. There may be situations where there is no clear route in the trips: for example, at the call of the heart, they got into the car and drove somewhere, unknown where, resting their soul and enjoying the beauty around. Or they know where to go, but on the road some kind of jumble arises: the navigator led the wrong way, there was some mix-up in the hotel.