Another useful interval for harmonizing melodies is the 3rd. Here is the E dorian scale harmonized in 3rds. The first example is on the first and second strings, while the second one is on the second and third strings.
Although the 6ths and 3rds shown in these last two lessons have been shown on only two sets of strings, it is important to learn them on each possible pair or strings. Once you know the sound of the harmonized scale and the notes contained in it, try working out the fingerings on the other strings. Here are some of the ways 3rds are commonly used.