found it ....
part # 3 is called (brake pressure modulator valve)..part of antilock brake system
"The Brake Pressure Modulate Valve is a part of your vehicle's anti-lock braking system(ABS).
It is a device that is controlled by the electric signal that is generated by the ABS module.
The brake pressure modulate valve regulates the pressure that is supplied to each ABS controlled brake in the wheels."
https://chevroletforum.com/forum/tah...id-part-78138/