When all three resistors are connected in series, the equivalent resistance is 6 + 6 + 6 =18, which is not the required number. Similarly, if all three resistors are connected in parallel, the equivalent resistor is 1/2

By connecting any two of the resistors in series or parallel, we may get the required value.

When two resistors are linked in parallel, the equivalent resistance of the two resistors is 3 ohm

The third resistor is connected in series with the first two then resistance is 9 ohm

The equivalent resistance of two resistors linked in series is given by

R = 6 + 6 = 12 

The third resistor is wired in series with 12 ohms. As a result, we can compute the equivalent resistance as follows:

R=4 ohm