In a parallel circuit, the total power supply current is the sum of the
currents through all of the individual branches.
The output current (not the voltage) of the power supply is 5 A.
So the individual currents in the circuit branches is 5 A.
The current through the first 2 resistors is (1 + 0.5) = 1.5 A.
So the current through the 3rd resistor is (5 - 1.5) = 3.5 A.