<form name="aaa">

<select name="test">

<option value="1">1</option>

<option value="2">2</option>

<option value="3">3</option>

</select>

<button name="test1" value="1선택" onclick="javascript:test('1');">

<button name="test2" value="2선택" onclick="javascript:test('2');">

<button name="test3" value="3선택" onclick="javascript:test('3');">

</form>

<script language="javascript">

function test(varRow)

{

  document.forms[0].test.options[varRow].selected = true;

}

</script>