"Groups of Three" is in reference to some of the pixel strips. Some strips come with one chip for every 3 pixels. You will see them referenced as 10/30 or the like. 10 nodes with 30 LED packages.

Bullet and square nodes are individually packaged with one chip per LED package. There are some rectangular nodes that also have one chip controlling 3 (or more) LED packages.

Usually we refer to a "pixel" as being a control chip and its associated LED packages. Some call them a "node".

Note: When I state "LED package", this is usually a red, green and blue LED inside a single carrier.