Wikipedia page https://en.wikipedia.org/wiki/American_flag_sort
I'm almost embarrassed by how long I've watched it!
Here's a visualization of a much larger data set that illustrates what's actually happening with the buckets and the major steps.
https://www.youtube.com/watch?v=21jEd1FUiV8
OT: I quite liked the follow up video Youtube suggested -
Sorting algorithms to relax/study to - https://www.youtube.com/watch?v=vr5dCRHAgb0
I'm almost embarrassed by how long I've watched it!