Connect an ammeter in the circuit and three voltmeter across each resistance as shown in figure. Note the reading of ammeter let us say it is
(i). Note the reading of voltmeters let us say they are V1, V2 and V3 respectively across R1, R2 and R3. Now by ohm’s law
We will find that i1 = i2 = i3 = i.
Hence same current flows through every part of the circuit.