anttenuator

why could 3 resistors be attenuator , either tee or pi type ?
what is its theory of attenuator ?
any reference i can read ?

The simplest thing I can think of is that it divides voltage from input to output.

for example you can consider the Tee model as a 2-port network and perform
ABCD analysis on it.

take the arms as R1=R2=8.56 ohm, and the shunt as R3=141.8 ohm.
you should get 3dB attenuation, i.e. only 0.707 volts are transferred to the output.

I recommend "Microwave Engineering", Ch.4 (Microwave networks) by David M. Pozar.

Good luck,
P.

Another aspect is that there are three degrees of freedom, input impedance, output impedance, and attenuation. It takes three resistors to control all of these three.