In a chord, the third factor is the note three scale degrees above the root (including the root itself in the count). For instance, in a C major triad, E is the third factor.
When the third is the bass note (i.e., the lowest note) of the triad or seventh chord, the chord is in first inversion.
Use
Conventionally, the third is third in importance to the root and fifth, with the third in all primary triads (I, IV, V and i, iv, v) being either major or minor.
The third in both major and augmented chords is major (E♮ in C) and the third in both minor and diminished chords is minor (E♭ in C).