How do you harmonize in thirds?
The traditional practice for harmonization in third intervals is that you harmonize a given note using another note that is a third below it. However in the case of the first tone of the scale we'll be going down a fourth instead.
What is the 3rd harmony?
Most chords in western music are based on "tertian" harmony, or chords built with the interval of thirds. In the chord C Major7, C–E is a major third; E–G is a minor third; and G to B is a major third. Other types of harmony consist of quartal and quintal harmony.