Getting Selection List To Work In IE And Firefox

This is part of a demo I'm doing in class of forms and using Javascript to validate. What's supposed to happen is this: when the user selects a choice, it's displayed in the field. If the user selects the right choice (Optimus Prime in this case) then a pop-up says "You win." The code works in Firefox but not in IE.

Form:
Code:
<form name="form3" onChange="javascript:checkChoice();">
Who is the coolest Transformer?
<select name="tf" size="1">
<option selected>Choose
<option value="Optimus Prime">Optimus Prime
<option value="Megatron">Megatron
<option value="Bumblebee">Bumblebee
<option value="Starscream">Starscream
</select>
<br><br>You chose
<input type="text" name="output3" value="" size=20 readonly>
</form>
checkChoice() function:

Code:
function checkChoice() {
var choice = document.form3.tf.selectedIndex;
document.form3.output3.value = document.form3.tf.options[choice].value;
checkData3();
}
checkData3() function:

Code:
function checkData3() {
if(document.form3.tf.selectedIndex==1) {
alert("You win");
document.form3.submit();
return true;
} else {
return false;
}}