Quest of the chord tones: the third

Quest of the chord tones: the third
What is a third and how do I find them on the neck?
How do I memorise easily where to find the buggers?
I found a Tom Bailey track with just three different chords and it sounds awesome. Bm G A and Bm again, this cycle repeats. Very good for this kind of video.
Then I play the tune and comment along the way the give you an idea what I am doing