JSFiddle - React, Tailwind, and code Playground

JavaScript

function unique(list) {
  var result = [];
  var counts = {};

  for (var i = 0; i < list.length; i++) {
    var c = list[i];
    if (!counts[c])
      counts[c] = 0;
    counts[c]++;
    if (counts[c] < 3) {
      result.push(c);
    }
  }

  return result.join("");
}

var srting = "DGHKHHNL";
var thelist = srting.split("")
console.log(unique(thelist));