Prøv så nedenstående. Kan testes på
http://eksperten.i.hasselager.net/javascript/20489b.htmMen det virker ikke i lidt ældre browsere (dog i IE4+ og NS6+)
<html>
<head>
<title>CheckBox</title>
<script language="JavaScript">
function selectx(nr,f) {
var c=0,i;
for (i=0;i<13;i++) {
eval('if (f.check'+i+'.checked) {c++;};');
}
if (c>=3) {
for (i=0;i<13;i++) {
eval('if (!(f.check'+i+'.checked)) {f.check'+i+'.disabled=true;}');
}
} else {
for (i=0;i<13;i++) {
eval('f.check'+i+'.disabled=false;');
}
}
}
</script></head>
<body bgcolor="#FFFFFF">
<form method="post" name="ff">
<input type="checkbox" name="check0" onclick="selectx(0,document.ff);"><br>
<input type="checkbox" name="check1" onclick="selectx(1,document.ff);"><br>
<input type="checkbox" name="check2" onclick="selectx(2,document.ff);"><br>
<input type="checkbox" name="check3" onclick="selectx(3,document.ff);"><br>
<input type="checkbox" name="check4" onclick="selectx(4,document.ff);"><br>
<input type="checkbox" name="check5" onclick="selectx(5,document.ff);"><br>
<input type="checkbox" name="check6" onclick="selectx(6,document.ff);"><br>
<input type="checkbox" name="check7" onclick="selectx(7,document.ff);"><br>
<input type="checkbox" name="check8" onclick="selectx(8,document.ff);"><br>
<input type="checkbox" name="check9" onclick="selectx(9,document.ff);"><br>
<input type="checkbox" name="check10" onclick="selectx(10,document.ff);"><br>
<input type="checkbox" name="check11" onclick="selectx(11,document.ff);"><br>
<input type="checkbox" name="check12" onclick="selectx(12,document.ff);"><br>
</form>
</body>
</html>