JSFiddle - React, Tailwind, and code Playground

HTML

<ul><li>A</li><li>B</li><li>C</li><li>D</li><li>E</li><li>F</li><li>G</li></ul>

JavaScript

$(function() {
    alert('Every third: ' + printObject( $('li:nth-child(3n)') ) );
    
    alert('Others: ' + printObject( $('li').not(':nth-child(3n)') ) );
});

function printObject( obj ) {
    return obj.map( function() {
        return $(this).text();
    }).toArray().toString();
}