$("#AFile #BFile #CFile")
will be AND operator in selectors which mean select a element with all 3 ids i.e. AFile
AND BFile
AND CFile
while
$("#AFile, #BFile, #CFile")
mean OR operator, which means select all elements with id as AFile
OR BFile
OR CFile
Just a side note in HTML element should have single and unique id. #AFile" && "#BFile" && "#CFile
(which means elements with id AFile AND BFile AND CFile) doesnt make sense in world of valid HTML
If you want to select all elements with id #AFile, #BFile
and #CFile
then you should use
$("#AFile, #BFile, #CFile).change(function() {
document.getElementById("UploadFile").submit();
});