var inputfirstdigits = document.getElementById("numberfirstdigits");
var inputmiddledigits = document.getElementById("numbermiddledigits");
var inputlastdigits = document.getElementById("numberlastdigits");


function nextbox(currentbox, nextbox)
{
var maxlength = currentbox.getAttribute("maxlength");
if(currentbox.value.length >= maxlength)
{
nextbox.focus();
}

}

inputfirstdigits.onkeyup = function(){nextbox(inputfirstdigits, inputmiddledigits);}
inputmiddledigits.onkeyup = function(){nextbox(inputmiddledigits, inputlastdigits);}
window.onload = function(){inputfirstdigits.focus();}