I was fortunate that my neighbors belonged to Medinah, so no. 3 was my first great course, at age 12, (after no. 2 and no. 1.....and no, that wasn't personal problem......, and one that convinced me I should become a gca.
Wracking my brain as to the second? I played Midlothian in high school, which is solid, but probably didn't experience another great course until after starting as an apprentice at Killian and Nugent, when we would play the courses around Chicago that we were consulting with, like Shoreacres, Bob O'Link, and a few others. And, for the first few years of my career, vacations were devoted to playing places like Olympic, Pinehurst, etc.
After I got into ASGCA, meetings focused (usually) on playing great courses in various metro areas, like Philly, NYC, Long Island, Pittsburgh, etc., so my experience on great courses picked up considerably.