Fast, palm muted “galloping” groups of 3’s on the open low E string are a staple of hard rock and metal rhythm playing.
These palm muted “gallops” earned their nickname by the way the sound like a horse galloping.
Though this rhythm approach is based around using notes in groups of 3, they’re not to be confused with triplets.
There are lots of different ways to play gallop style rhythms, but a lot of them don’t use triplet rhythms.
Instead the 1st note of the group of 3 is played on the beat as a 16th note, the 2nd note of the group of 3 is played on the “e” of the beat as a 16th note and the 3rd note of the group of 3 is play on the “&” of the beat as an 8th note.
The best way to improve this rhythm technique is to practice it along to a metronome at a slow tempo you can handle.
Take the tempo up as you become more and more comfortable with it.