Watch the 3rds online guitar lesson by Charles Chapman from Interval Insights
Thirds are the easiest intervals to hear and finger. Most styles of music are inundated with them.
Pay careful attention to the fingering and the string sets in the exercises. Make sure you can execute them flawlessly before you alter fingering or positions.
Many consider the 3rd the matriarch in the interval family - the "mother of all intervals." If you construct four-note chords in 3rds you will get the diatonic harmonic structures for all major keys. This sets up the basis for chord analyzation and is the basic foundation for most contemporary harmony (see the diatonic chords of C Major in the PDF).
1- Ascending 3rds Played Single
2- Descending 3rds Played Single
3- Simultaneous 3rds
4- Ascending 3rds Played Single Simultaneous
5- Descending 3rds Played Single Simultaneous