Watch the Triplet Thumb online guitar lesson by Stu Hamm from Stu Hamm U: Slap Bass
Once you get comfortable with the "double thumb" stroke, let's add another stroke to make it a triplet!
You are going to have to keep your wrist a bit tighter than when we were bouncing our thumb off the strings. Here the thumb is really acting as a plectrum so it needs to be secure.
This is also a technique that flamenco guitar players use, so once you get the hang of it you can really create some interesting rhythmic patterns.