PCM
Spring, Texas
Another way to think about it is that you aren't shifting a box 3 frets, you're shifting 1 box up or down (minor to major...major to minor respectively) from the same root/anchor/position to get a different scale (major or minor).In "Slow Blues Supplement" and "Major Minor Blues Shapes" Griff discusses some of the following concepts.
In SBS at least check out:
0:00-3:15 Introduction
0:00-3:10 Lesson 1 (Relative keys)
0:00-1:00 Lesson 2 (Root notes as anchors)
4:43-7:10 Lesson 2 (Right facing-Left facing)
In MMBS, the Intro: Mental gymnastics of "shifting" boxes 3 frets and Roots as "pivot points"
Memorizing the "boxes" is one thing. How to manipulate them when playing is another entirely. The latter is a lifetime exercise.