<form id='alta' name='nalta' method="post" >
<div id="caja1">Caja1</div>
<div id="caja2">Caja2</div>
<div id="caja3">Caja3</div>
<label>Cajas</label>
<select id="select" onchange='cajas()'>
<option value="1">uno</option>
<option value="2">dos</option>
<option value="3">tres</option>
</select>
<label>Efectos</label>
<select id="efecto" name="efecto" onChange="efectos()">
<option value="c">color</option>
<option value="b">background</option>
<option value="t">border</option>
</select>
<label>Accion</label>
<select id="accion" name="accion" onChange="accion()">
<option value="b">blue</option>
<option value="o">orange</option>
<option value="500">50px</option>
</select>
</form>
function cajas() {
var x=0;
x=eval(document.getElementById('select').value);
if (x==1){
document.getElementById('caja1').style.border ='2px solid red';
document.getElementById('caja2').style.border ='none';
document.getElementById('caja3').style.border ='none';
}
else if (x==2){
document.getElementById('caja2').style.border ='2px solid red';
document.getElementById('caja1').style.border ='none';
document.getElementById('caja3').style.border ='none';
}
else if (x==3){
document.getElementById('caja3').style.border ='2px solid red';
document.getElementById('caja1').style.border ='none';
document.getElementById('caja2').style.border ='none';
}
}