This is Nick’s question. Nick is an amazing person and I will try to answer. Mercury in the 3rd House would be a person who needs to communicate. This native could not have a relationship without simpatico. He, also, desires self expression. For some people, it is more important than to others. The 3rd House is Gemini’s House. Geminis need to communicate in the same way that humans need to breathe air. I am a Gemini. Self expression is my top personal need.When I express myself, I am happy. This expression can be written, verbal or even creative such as singing and playing an instrument. The expression must be authentic. A Gemini with a socially appropriate, politically correct mask will be a very unhappy person. The key phrase for Gemini and Mercury in the 3rd House is that authentic communication makes for happiness.
Mercury in the 12th is a horse of a different color. This native fears communication. Every living being craves intimacy. Authentic communication is the foundation for intimacy. The 12th House native is scared to death of saying the wrong thing. This native may hem and haw before he, even, takes a chance on expressing an opinion. He may have been humiliated for expressing opinions. Any planet in the 12th gets suppressed. The native develops a fear of sharing himself in the domain of the planet. For Mercury, it is communication.
What can a 12th House Mercury do to remedy this? The Astrologer must look for compensating factors in his chart. For a 12th House Mercury, Jupiter in the 3rd House would be one compensating factor, for an example.