function goInputBox2(){
var idx1 = document.getElementById('corpNum1');
var idx2 = document.getElementById('corpNum2');
if(idx1.value.length == 3){
idx2.focus();
}
}
<input id="'corpNum1" type="text" onkeyup="goInputBox2()"/>
function goInputBox2(){
var idx1 = document.getElementById('corpNum1');
var idx2 = document.getElementById('corpNum2');
if(idx1.value.length == 3){
idx2.focus();
}
}
<input id="'corpNum1" type="text" onkeyup="goInputBox2()"/>