Three notes being played simultaneously are referred to as a Triad.
Triads are the building blocks of Chords.
The most common triads in popular music are the major triad and the minor triad.
Major triads are derived from the major scale, and are the basis of the major chord. As an example, let's look at the A major scale:
The A major (or A) triad is derived from playing the root (1st = A), the 3rd (= C#) and the 5th (= E) notes of the A major scale together.
So if we play A, C# and E simultaneously, we will produce the chord known as A major, (normally referred to as just 'A'). Here are some ways we could play the A major triad:
The third note is referred to as a Major Third, since this is the note that determines whether a chord is major or minor.
For example if you wanted to know C major, you would find that it contains the notes C(1st), E(3rd) and G(5th) from the C major scale.
For D (major) you would use the notes D(1st), F# (3rd) and A (5th).
For E (major) you would use the notes E(1st), G# (3rd) and B (5th).
For F (major) you would use the notes F(1st), A (3rd) and C (5th).
For G (major) you would use the notes G (1st), B (3rd) and D (5th).
For B (major) you would use the notes B (1st), D# (3rd) and F# (5th).
When applying these to the guitar, if you are familiar where all the notes lie across the fingerboard (which you should be), you can construct any major triad in many different positions (or voicings).