var str = "haaaaaaaaaaallllllllllllllllllllllloooooooooooooo";
var strs = str.split("");
var j = [];
for(var i in strs)
{
    if(strs[i] === strs[i-1] && strs[i] === strs[i-2]) j.push(i);
}
j.reverse();
for(var i in j)
{
    strs[j[i]] = "";
}
console.log(strs.join(""));