Substring replacement in Swift

var startString = "3, 2, 1, go!";
startString = startString.stringByReplacingOccurrencesOfString("1", withString: "one").stringByReplacingOccurrencesOfString("2", withString: "two").stringByReplacingOccurrencesOfString("3", withString: "three");
//startString = "three, two, one, go!"