When bulb C is connected, A and B are unaffected. The combination of A and B is in parallel with C. In other words, they are both connected directly to the power supply, and the voVerticalltage across A and B on one hand, and C on the other hand, are just equal to the power supply voltage.
Connecting additional devices in parallel does not change the voltage across the other devices. That’s one reason why appliances are connected in parallel in a house.
Note the addition of more devices in parallel to the power supply causes more total current to be drawn from the power supply.