#3 is correct for the waiter situation. “I’ll be right with you” means “I will the available to help you in a very short time” (usually, less than a minute or two). #1 is usually used when somebody contacts you for information or a decision (as opposed to seeking some kind of personal service, such as the waiter cleaning up a broke class), and you are not able or willing to immediately provide it. “I’ll get back to you” implies that you will respond eventually, but it may take hours, days or weeks. The meaning of #2 depends on context, but it usually be used when you are going away physically from somebody (like going to another city) and are promising that you will return. Example, spoken to a small child: “Don’t worry, Mommy’s just going to work. I’ll come back to you before dinner.”