Well, the main difference is that the last two don't sound right. You should have a verb after "so as" and "in order to". Here are some examples: He raised his voice in order to (make sure that) the rescuers would hear him. He raised his voice so as (to ensure that) the rescuers would hear him. You could also put in other verbs. Does that help? Other than that, they seem pretty much synonymous to me in this case.