Watch the Using 3rds online guitar lesson by Jeff McErlain from Blues Guitar Survival Guide - Lead
Certainly there are many intervals we can play on the guitar but 3rds and 6ths and essential to any blues player. Both these intervals are contained in any chord. Interestingly enough a 3rd which is the building block of western harmony when inverted becomes a 6th. For example the notes G to B is a major 3rd (two whole steps) but B to G is a major 6th. The reason why the 3rds and 6ths sound so comfortable is that we are using the most tonal interval we have. It is very useful to see what notes sit best on each chord. The ones that sound best I will bet are the ones that contain the chord tones.