Check if specific checkboxes are checked

I have a form with multiple checkboxes like this:

<form id="myForm">
   <input type="checkbox" id="first" />
   <input type="checkbox" id="second" />
   <input type="checkbox" id="third" />
</form>

I want to do something (i.e. alert box) only when checkbox 1 and 3 are selected.

I know how to target one check box like this:

$('#first').click(function () {
    if ($(this).prop("checked") == true) {
        alert("I'm checked!");
    }
});

But is there a way to do that for specific multiple checkboxes?