Here is a 2-octave major scale with the scale degrees numbered 1 through 15:
3rds are created by stacking two notes of the scale that are a 3rd apart: 1 and 3, 2 and 4, 3 and 5 and so on. You don't have to be able to read music to get this concept; here's Scale Tone 3rds in the key of C with the scale degrees written under them:
There are two kinds of 3rds with which we're concerned here; a Major 3rd and a Minor 3rd. The Major 3rd consists of 2 whole-steps. The Minor 3rd consists of a step-and-a-half.
A half-step on the guitar is moving from one fret to the adjacent fret:
A whole-step has a fret in-between:
Obviously, you can't play 2 notes on the same string at the same time so we will play the 2 whole steps (Major 3rd) and the step-and-a-half (Minor 3rd) on adjacent strings:
Major Scale Formula:
In order to run 3rds along the scale, you must be able to visualize the scale along a single string. The key to being able to do this is knowing the major scale formula. That is, the sequence of whole-steps and half-steps in the scale. The trick is remembering that there is a half-step between 3 and 4, and also 7 and 8; everything else is a whole-step:
Now we're going to play scale tone 3rds. The blue notes are the roots and they follow the major scale formula; notice the half-steps from 3 to 4 and from 7 to 8:
Drill this pattern ascending and descending - both directions.
Add 3rds to the Scale
When adding 3rds to the scale, the 3rds based on 1, 4, 5 and 8 are Major and 2, 3, 6 and 7 are Minor:
Drill Scale Tone 3rds ascending and descending. Always be cognizant of what scale degree you're playing; that tells you whether the 3rd is Major or Minor and it tells you the interval (half-step or whole-step) to the next note in the sequence
1 is Major, a whole-step to 2 which is Minor, a whole-step to 3 which is Minor, a half-step to 4 which is Major, a whole-step to 5 which is Major, a whole-step to 6 which is Minor, a whole-step to 7 which is Minor, and a half-step to 8 which is Major.
3rds on Strings 2 & 3
On strings 2 and 3, the shapes are different:
Scale Tone 3rds on the 2nd set of two strings - strings 2 and 3:
Drill Scale Tone 3rds on strings 2 and 3 ascending and descending. Always be cognizant of what scale degree you're playing.
A huge percentage of 3rds in the real world occur on the 1st two sets of 2 strings; strings 1 and 2 and strings 2 and 3. Therefore, commensurate with the "Quick & Dirty" concept, those are all that we'll cover here.
In order to put Scale Tone 3rds into keys you have to know the name of the root note. We're not going to go into how to do that here; that's covered in the 5-Lesson Foundational Series. There is a free webpage-based version here - but you must be a Site Member and logged in to view it.
Nevertheless, you can drill Scale Tone 3rds without knowing keys, just pick a note on the 2nd or 3rd string to serve as the 1 and go from there.
An example of the use of 3rds in a song would be the hook line to Van Morrison's Brown Eyed Girl. A demonstration is at the end of the video below.
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