Limit the maximum of currently selected options for a selectbox

selectbox-limit-max-selections.js

(function() {
    var verified = [];
    document.querySelector('#myselector').onchange = function(e) {
        if (this.querySelectorAll('option:checked').length <= 3) {
            verified = Array.apply(null, this.querySelectorAll('option:checked'));
        } else {
            Array.apply(null, this.querySelectorAll('option')).forEach(function(e) {
                e.selected = verified.indexOf(e) > -1;
            });
        }
    }
})();