var arr = ['a', 'b', 'c', 'c', 'd','e', 'e', 'e', 'e', 'e', 'f', 'e', 'f', 'e', 'f', 'a', 'a', 'a', 'f', 'f', 'f'];
var last = '';
var second_last = '';
var opened = false;
var res = "";
for(var i in arr) {
	if(arr[i] == last && last == second_last && opened == false) {
		res += "<span>";
		opened = true;
	}
	if(opened && arr[i] != last) {
		res += "</span>";
		opened = false;
	}
	
	second_last = last;
	last = arr[i];
	
	res += arr[i];
}
if(opened) res += "</span>";
alert(res);