I have a feeling what your after is if I press this switch and it's load turns on I want all these devices to turn on aswell?
If that's the case groups do work but you need more than that. Groups are a group of devices which you want to control at the same time like a chandelier with 5 RGB lamps in for example. That would create a new RGB device and when you turned on the group device all 5 of the physical devices would turn on.
So in your case you could use a group for all your switches but you need something to tell that group to turn on. Now if you're using a switch controlling a load you could do it a few ways depending on what the switch does.
For example let's say when you press the switch it turns on the light but also it creates a button event IE button one pressed. You could then create a button rule that said when button 1 is pressed from switch A, B or C turn on group 1.
Then the same for off but button 2 and turn off group 1.
If your switch doesn't do that you could create a rule that says triggers of if switch A, B or C turn ON.
Actions of turn on group, wait for event of A, B or C turned off and turn off group.
There is also a mirror app Which may do something similar.