How did I know even before clicking this was going to be a depression or ADHD issue. Seems this is getting to be a daily post.
So, I went to the psychologist. He diagnosed me with a "dysthymic disorder" which is basically mild depression that comes and goes.
It's true there is a question that asks you to list medical visits in the past three years.
But, question 18m asks "Have you EVER been diagnosed mental disorders of any type; depression, anxiety, etc"
The three year thing means you MUST list things in the past three years, but doesn't mean you MUST NOT list anything prior to 3 years.
You can lie on the form and risk getting caught. As a hobbiest, you'd likely just lose your license. As an airline pilot, you'd likey face criminal charges; lying on the medical form is a felony.
If you chose to confess, seek the assistance of an AME who specializes in getting depression, ADHD, anxiety past the FAA (Dr. Chien - google him), before filling out any paperwork or actually doing a medical exam. From the information provided, it seems like you'd get through this with time, money, and patience.