<form method="post" name="myForm" action="">
<select onChange="this.form[1].selectedIndex = this.form[2].selectedIndex = this.selectedIndex;">
<option>Яблоко</option>
<option>Груша</option>
<option>Тыква</option>
</select><hr />
<select>
<option>Яблоко</option>
<option>Груша</option>
<option>Тыква</option>
</select><br />
<select>
<option>Яблоко</option>
<option>Груша</option>
<option>Тыква</option>
</select>
</form>