Sorry i missed on the circulator part. Its functionality is similar to both of the afore mentioned components to some extent. I dont know the MAJOR difference between the three but i feel all can serve you in similar way off adding signals to existing signals. You can think of circulator as an adder too. As far as i am concerned, i fell add module and circulator have almost similar functions but somehow it depends on your system implementation and how you are using both in your system design. I hope it is helpful.
Regards