JSFiddle - React, Tailwind, and code Playground

JavaScript

const string = 'stackoverflow';
function spliter(nr) {
    const chars = nr.split('');
    let temp = [];
    let parts = [];
    for (let char of chars) {
        temp.push(char);
        if (temp.length == 3) {
            parts.push(temp.join(''));
			temp = [];
        }
    }
    if (temp.length) parts.push(temp.join(''));
    return parts;
}

console.log(spliter(string));