function CheckCheckbox(checkb){


switch (checkb){
case 1:
document.forms[0].check2.checked = false;
document.forms[0].check3.checked = false;
break;

case 2:
document.forms[0].check1.checked = false;
document.forms[0].check3.checked = false;
break;

case 3:
document.forms[0].check1.checked = false;
document.forms[0].check2.checked = false;
break;


}


}

</script>

aanroepen met:

<input type="checkbox" name="check1" id="check1" onclick="CheckCheckbox(1)"/>