3 notes per string
If you take that pattern for a major scale of Tone Tone Semitone Tone Tone Tone Semitone it means that there are only 3 ways that three notes can be spaced apart by these tones and semitones: Tone Semitone, or Semitone Tone, or Tone Tone.
They look like this:
Naming the 3 note shapes
The label above each diagram is a way of simplifying how you think about these shapes. In every shape you will play with the first and fourth fingers. So it is easiest to call the one where you use your third finger ‘3’. The difference between the ‘2’ and ‘stretch’ shapes is that in the stretch shape you just stretch your finger away one fret to give a tone between your first and fourth fingers.
It cannot be stressed enough that you must stick to this fingering at all times. Don’t use your third finger for the stretch, it makes things more difficult because your third finger is the weakest finger. If your posture is right then it is far easier to stretch between fingers 1 and 2 than between 3 and 4 – even if it’s just because the 4th finger is shorter.
If you vary the fingering then you are multiplying the amount of work your brain has to do to remember the shapes.