Counting by \(3\)s means going up by 3 each term instead of by \(1\).

Here is usual counting: \[1,2,3,4,\dots\]

Here is counting by \(3\)s: \[3,6,9,12,\dots\]

In regular counting by \(1\)s you can start from any number. Here is counting by \(1\)s starting from \(8.\) \[8,9,10,11,12,13,\dots\]

Here is counting by \(3\)s starting from \(8.\) Each term goes up by \(3.\) \[8,11,14,17,20,23,\dots\]