How to uncheck the checkbox in AJAX?

Seriously i have tried everything that i could think of but i still cant find a way how to uncheck the checkbox once popping up the alert.

var sessionvar;
            $.ajaxSetup({cache: false})
            $.get('test.php' ,function (data) {
                sessionvar = data;
                alert(sessionvar);
                if(checked){    
                    if(sessionvar > 2){
                        alert('You only allowed to select maximum 3');
                        setsession(sBorrowValue, "UNSET");
                        $(this).attr('checked', false);
                    }
                    else{
                        setsession(sBorrowValue, "SET");
                    }
                }
                else{
                    setsession(sBorrowValue, "UNSET");
                }
            });

I want to uncheck the checkbox after the alert('You only allowed to select maximum 3'); but not working. i also tried using this.checked = false, $(this).attr('checked', false);, $(this).prop('checked', false); but still its not uncheck the checkbox.

NOTE: this.checked = false works when outside from ajax. but when i put it on the inside, its not working.