I don't understand the need for a buzzer. Also, if there are only 3 signals, it is probably a pretty small waiting area. Why not handle it in a more personalized manner? Most patients appreciate a personalized approach.
If this is really a model that you think can be scaled up, then consider some of the problems of scaling up such an LED/buzzer system in a waiting room with 25 to 100 patients. I am assuming of course that this project is for real or should emulate a real situation. That is, it is not just an assignment to light 3 LED's and give a short pulse when each is lit.
If it is the latter, you don't need to control the monostable power, just use the trigger/reset and don't allow retriggering.
If it is for a real situation, then there is a lot more to discuss. Start with how big is the room, how big a practice, how many patients per hour/day, how many examining rooms, is it for specimen collection, nurse exams, or physician exams? Do you have to deal with regulations like HIPAA in the USA? Is this a system for telling the doctor or other care provider that a patient is in the examining room, or is it to notify the patient the patient is is "his turn?"
John