SOURCE

var ajax1 = $.ajax({
  url: 'xx.php',
  data: {}
});
var ajax2 = $.ajax({
  url: 'xxx.php',
  data: {}
});
var ajax3 = $.ajax({
  url: 'xxxx.php',
  data: {}
});
$.when(ajax1, ajax2, ajax3)
  .done(function(a, b, c) {
  console.log(a,b,c)
  //dosomething
})
  .fail(function(a, b, c) {
  //dosomething
});