<button type="button" class="calculate">Calculate</button>
    <script>
        $(document).on('ready', function() {
            $(".calculate").on('click', function() {
            var x1 = {{ var1 }};
            var x2 = {{ var2 }};
            var x3 = {{ var3 }};
            var total = (x1 + x2 + x3)/3;
            $('p.hui').html(total);
            });

        });
    </script>
<p id='hui'></p>