Google - Prove that the resultant restaurant of three restances connected in parler?

1. Do not preface your question with "Google" - that is just silly.

2. Learn the words so you understand what you are studying:
"Prove that the equivalent resistance of three resistances connected in parallel
1/R = 1/R1 + 1/R2 + 1/R3."

3. Proof: Ohm's law defines resistance as voltage divided by current, R = E/I.
Kirchoff's law says the sum of currents into a point equals the sum of currents out of the point It = I1 + I2 + I3
Assume the voltage is one volt. Therefore 1/R = 1/R1 + 1/R2 + 1/R3.

Here is a free course in basic electronics: www.phy.davidson.edu/instrumentation/NEETS.htm