function checkall(form) {
    var subBtn;

    for (i = 0; i < form.elements.length; i++) {
        if (form.elements[i].type == "submit") {
            subBtn = form.elements[i];
        }
    }

    var chk1 = document.getElementById('checkbox0');
    var chk2 = document.getElementById('checkbox1');
    var chk3 = document.getElementById('checkbox2');

    if (chk1.checked == true && chk2.checked == true && chk3.checked == true) {
        subBtn.disabled = false;        
    } else {
        subBtn.disabled = true;
    }
}

function disableSubmit() {
    var subBtn;

    for (i = 0; i < document.forms[0].elements.length; i++) {
        if (document.forms[0].elements[i].type != null && document.forms[0].elements[i].type == "submit") {
            subBtn = document.forms[0].elements[i];
            break;
        }
    }
    if (subBtn != null) {
        subBtn.disabled = true;
    }
}