How will you conclude that the same potential difference (voltage) exists across three resistors connected in a parallel arrangement to a battery?
Make a parallel combination, XY, of three resistors having resistances R1, R2, and R3, respectively. Connect it with a battery, a plug key and an ammeter, as shown in figure.
Now, connect a voltmeter in parallel with the combination of resistors. Plug the key and note the ammeter reading. Let the current be I. Also take the voltmeter reading. It gives the potential difference V, across the combination. The potential difference across each resistor is also V. This can be checked by connecting the voltmeter across each individual resistor.
Take out the plug from the key. Remove the ammeter and voltmeter from the circuit. Insert the ammeter in series with the resistor R1, as shown in figure:
So if you note all the voltmeter readings in each parallel resistor it will be the same.