var enteredText = "";
function changeKey()
{
	var element = document.getElementById("my_element");
	symbolCode = (document.all) ? event.keyCode : arguments.callee.caller.arguments[ 0 ].which;
	enteredText += (String.fromCharCode(symbolCode)).toLowerCase();
	for (i =  0 ; i < element.options.length; i++) {
		foundedText = (element.options[i].value.slice( 0 , enteredText.length)).toLowerCase();
		if (enteredText == foundedText) {
			element.selectedIndex = i;
			return true;
		}
	}
	if (enteredText.length ==  3 ) enteredText = "";
}