What of the relative minor scale? There are three major triads there also. Following an identical process and using a major triad ‘measuring tool’ would show this.
The C major scale and the A minor scale are relatives to one another. To view it in a similar way and use our measuring tool, all we need do is slice off one edge and move it to the opposite side.
Root note A
As before, the shape does not match our tool, it is an A minor triad.
Root note B
As before, the shape does not match, it is a B diminished triad
Root note C
As before, this shape is an exact match, it is a C major triad.
Root note D
As before, the shape does not match our tool, it is a D minor triad.
Root note E
As before, the shape does not match our tool, it is an E minor triad.
Root note F
As before, this shape is an exact match, it is an F major triad.
Root note G
As before, this shape is an exact match, it is a G major triad.
As we now know and expected, there are three major triads.
I hope the additional information and explanation help give some of the how and why.
Cheers 
| Richard | JustinGuitar Approved Teacher, Official Guide & Moderator
Can you spot the mistake? 
1 Like
It’s a minor typo which turns it into a major error 
2 Likes
Whoops.
All corrected now, thanks Jeff.
Minor crisis averted!
2 Likes