Convert the sources into their Norton equivalent. First convert the three current sources to their equivalent voltage sources. Using V=IR find the equivalent voltage value and connect it in series with its respective resistor. You will get the second circuit as shown. As all of them are in series add the voltage sources and the resistors to get the equivalent circuit as shown made up of one voltage source and resistor. Convert this to a current source and resistor in parallel to it as shown. Note that the voltage across the terminals remain same in all cases.