In a parallel circuit with 3 resistors, the output voltage at the power supply is recorded to be 5 a. the current flowing through the first resistor is 1 a and the current flowing through the second resistor is .5 a. what is the current flowing through the third resistor?

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.