Prueba con
<form name="form1" method="post" action="">
<p>
<select name="select" onchange="this.form.textfield.value = this.value">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
<input name="textfield" type="text" value="">
<input type="submit" name="Submit" value="Submit">
</p>
</form>
