All  chords are formed with thirds. Major and minor thirds are the building blocks of harmony. Two of them will combine to create four different triads. A minor- third is 3 semitones, and a Major- third is 4.  When combined, their total is either 6, 7, or 8 semi-tones. This creates either a diminished, perfect, or augmented fifth. Any three notes have a sound, but unless they have this structure they are not chords. These four triads shown below provide the foundations of all our chords.
The diagram below shows the triads on only three strings.
These are all the possible positions for a major triad.
In the diagram below, a minor  third replaces the major third, showing all the positions of a minor triad.
Repeat the exercises described above, refingering the positions for the lowered third.