I said "in reading greens."
A 3' level would likely span too much of an area and the slope would likely change a little. As I said, one end or the middle might be elevated slightly because of a valley or ridge. Greens aren't perfectly planar. A 9" level is a good compromise: big enough to get an accurate read, small enough not to span bumps and valleys.
Take an AimPoint class.
These posts remind me of a story I read a long time ago that may be apocryphal. In my mind "apocryphal" translates to, "if it's not true, it oughta be!" According to the story Joe Dimaggio and Sam Snead were at some event where they got into a discussion about the difficulties of their sports. After a lengthy dissertation by Dimaggio about how hitting a baseball was the hardest thing in sport Snead had a reply. He said, something like, "Everything you say is true Joe, but you don't have to climb up in the stands and play your foul balls!"
No doubt, some of these young buck baseballers can hit the ball a long way, But that's basically all they do. Try to hit it hard and "put the ball in play". In a field of play that's about as wide as ten fairways in golf. When it comes to finesse and touch, they have nothing!
There have been quite a few ex-pro baseball and football players who have becomes quite good at golf. But, how many of them have really threatened joining the pro ranks? That tells you right there. Each sport is it's own endeavor, and ALL the skills don't necessarily translate.