You could put this code into your exit event of you drop-down list.. Just change TextField1 to your field's name and make sure the Language is set to JavaScript:
if (this.selectedIndex == 2){
TextField1.presence = 'visible';
TextField1.mandatory = "error";
TextField1.mandatoryMessage = "This field is mandatory!";
}
else {
TextField1.presence = 'hidden';
TextField1.mandatory = 'disabled';
}