Norton equivalent circuit question
Your question appears to be flawed because it is asking for the Norton equivalent of a current source in parallel with a resistor and that is already the Norton equivalent: -



So unless I'm getting confused, I believe your question is probably meant to be asking for the Thevenin equivalent.

If the open circuit voltage is 30 volts and the current is 3 mA the value of the resistor is 10 kohm and this is equivalent to a voltage source of 30 volts in series with 10 kohm.


  I'm not sure if the current through the shorted branch ab would be the
  same as the current from the source.


It's a current source and the shorted current will be 3 mA.