If R1, R2, and R3 have all different values, which resistors can you be sure have the same potential difference falling across them?

The resistors in parallel.

Explanation:

If you have 2, 3 or infinite resistors in parallel (that means when both of their terminals are respectively connected to each terminal of the others resistors), they all have the same potential difference even when they have different values. Of course that means if the resistor value increases, the current decreases since V=I. R, where V is a constant in the parallel resistors.