mmm... se me ocurrió esto...
Código HTML:
<form name="formulario" method="post" action="">
<select name="s1" onChange="cambiaS1(this.value)" >
<option value="0">NO ASIGNADO</option>
<option value="a">Persona A</option>
<option value="b">Persona B</option>
<option value="c">Persona C</option>
</select>
<select name="s2" onChange="cambiaS2(this.value)" >
<option value="0">NO ASIGNADO</option>
<option value="a">Persona A</option>
<option value="b">Persona B</option>
<option value="c">Persona C</option>
</select>
<select name="s3" onChange="cambiaS3(this.value)" >
<option value="0">NO ASIGNADO</option>
<option value="a">Persona A</option>
<option value="b">Persona B</option>
<option value="c">Persona C</option>
</select>
</form>
Código PHP:
<script language="JavaScript" type="text/javascript">
function cambiaS1(valor)
{
if (document.formulario.s2.value==valor)
{document.formulario.s2.value=0;}
if (document.formulario.s3.value==valor)
{document.formulario.s3.value=0;}
}
function cambiaS2(valor)
{
if (document.formulario.s1.value==valor)
{document.formulario.s1.value=0;}
if (document.formulario.s3.value==valor)
{document.formulario.s3.value=0;}
}
function cambiaS3(valor)
{
if (document.formulario.s1.value==valor)
{document.formulario.s1.value=0;}
if (document.formulario.s2.value==valor)
{document.formulario.s2.value=0;}
}
</script>