Voltage divider ambiguity
Yes, potential difference is a function of the current flowing and the resistance it is flowing across. 


  V = I * R




Thus, since point B is a hypothetical open circuit, current through R3 is zero, hence voltage across R3 is also zero.

As an aside, while the ascii-art schematic is better than none, there are a number of simple, and free, schematic drawing tools out there, such as Fritzing, for making a schematic that provides greater clarity to the reader of your question.