Albeit badly drawn and I cannot make any sense of it, what you have is a Thevenin circuit. The current will be derived from shorting the output of the circuit. Its equivalent impedance looking in will be the equivalent resistance with the current source removed.
Remember, Thevenin voltage sources have zero impedance/resistance (short circuit), Norton current sources have infinite impedance/resistance (open circuit). This refers to the actual voltage/current source and NOT the circuit.
So based on the circuit by DerStrom8 you have a voltage source (remember without any load whatsoever) of V1 * R2/(R1+R2) and an internal resistance (remember a voltage source has zero impedance so short it out for analysis) or R3 + R1//R2.
R3 doesn't come into the voltage calculation as a Thevenin analysis requires you are drawing no current so there is no voltage drop across R3.
You can convert this to a Norton by shorting the output giving a current source of V1/(R1+(R2//R3)) and the rest I am sure you can work out.