Can people be trusted? If you answer this question on a basis of your relationship with a Neptune in 3rd House synastry partner, the answer will likely to be “Better not.”
Even if she is a very honest and reliable person in general, you two will often have mutual misunderstandings, and it will seem that she always misinterprets your words and actions. At the same time, under the influence of her Neptune, your Third House will blow up: for example, in her presence people will keep calling your phone number by mistake, or your appointments and arrangements will be cancelled. All this, however, will be quite perfunctory, unless her Neptune is in synastry with more than two planets in your Third House. At the same time, the influence of your Third House on your partner’s Neptune can be much stronger.
For example, if the boss’ Neptune is in the Third House of his secretary, he (in a harmonious relationship) can feel happy at the mere sight of her dialing a phone number to connect him with a supplier, which she may not even suspect that being constantly upset with his absent-mindedness in conversations with her even on crucial issues. The boss will probably notice that he works better when this secretary is not present but is unlikely to attach much importance to this, considering it a pure coincidence.
In general, Neptune in 3rd House synastry provides great opportunities for mutual deceit, so the partners must handle any information with great care and responsibility.
from A. Podvodny
Neptune in 3rd House Synastry Explained
Neptune in 3rd House Synastry forms fragile and ambiguous psycho-emotional relationships between people on the subconscious level. Various problems of mutual understanding can arise because the “Neptunian” person relies on the psycho-intuitive abilities of imagination while the person on the Third House relies on the logic and the objectivity of information.
A lot depends on the mental characters of the partners reflected in their natal chart. If the aspect is positive, they find many common interests in the spiritual, religious, cultural, and intellectual spheres of life. At the same time, the intuitive abilities of the “Neptunian” personality are perfectly combined, constructively and creatively, with the practical skills of the Third House personality.
When Neptune in 3rd House Synastry is negative, the partners have problems and difficulties in relationships related to absent-mindedness and insincerity of the “Neptunian” nature who considers the person of the Third House to be incapable of intuitive thinking, attached to trifles, pedantic and a down-to-earth person who, in turn, considers the “Neptunian” personality simply unbearable.
Nevertheless, Neptune in 3rd House Synastry is generally considered positive as these relationships can be promising in those areas of life that are associated with creative and spiritual-intellectual activity, as well as marital life.
from F. Sakoyan, L. Ecker