in a two transistor led flasher. why does r3 controll the leds brightness and limits the current flow?

Answer 1

You need to give a link to the circuit diagram.

Answer 2

Any led uses a current limiting resister. It is not the resister that controls the brightness. That is a function of the control circuit. Even though an led is a current operated device, the Vcc is what controls it's brightness. This is how battery chargers know when to turn on the charge indicating led to show full charge has been obtained. As the internal Resistance of the battery changes, the loaded charger goes up in Vcc till the led lights.