addClass every nth
try
$("ul li:nth-child(3n+1)").addClass("A")
$("ul li:nth-child(3n+2)").addClass("B")
$("ul li:nth-child(3n)").addClass("C")
Feel free to consolidate it to make it prettier, but I wanted to expose the selectors.
I recommend something like this:
let i = 0;
$("li").each(function() {
i++;
let newClass = 'A';
if (i % 3 == 1) { newClass = 'B'; }
if (i % 3 == 2) { newClass = 'C'; }
$(this).addClass(newClass);
});