agert,
Вариант ...
<script type="text/javascript">
var a,b;
function foo(c) {
if (a != c) {b = 0;a = c};
b ^= 1;
c.checked = b
};
</script>
<input type="radio" name="r" value="1" onclick="foo(this)"/>
<input type="radio" name="r" value="2" onclick="foo(this)"/>
<input type="radio" name="r" value="3" onclick="foo(this)"/>