<input name="test1" value="1" onclick='vstavka(this)'>
<input name="test1" value="2" onclick='vstavka(this)'>
<input name="test1" value="3" onclick='vstavka(this)'>
<br><br>
<input id="go" value="">
<script>
function vstavka(ths) {
document.getElementById("go").value = ths.value;
}
</script>
как вариант)