Answer
Potential difference between points 1 and 2 $\Delta V_{12}$ will be equal to 3$V$
Work Step by Step
Since the points 1 and 2 are not connected it means circuit is open and there is no current in the circuit. As there is no current there will be no potential drop across resistor and potential and 1 will be 3$V$ and potential at point 2 will be 0$V$.
thus potential difference between points 1 and 2 will be $3V -0V = 3V $
