Replace any 3 letters from the string with random letters

I want to replace any 3 random letters from the string with random letters from my letters variable

var str = "HELLO";
var letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
var arr = str.split('');
for (var i = 0; i < 3; i++) {
    var pos = Math.round(Math.random() * arr.length - 1);
    arr.splice(Math.floor(Math.random() * arr.length), 1);
    str = arr.join('');
}
alert(str);

I am able to take 3 random letters out right now but can't figure out how to get 3 random letters from letters and put them in a random position.

Here is the demo of what I have right now.

http://ift.tt/1HM36zk

Any help would be appreciated!