Let the total resistance is Req and dc voltage is V voltage and total current be I amp then.

Total power P = 72W and I = 6A

V*I = 72

Then V = 12V

Current through first resistor is I1 3A let it resistance is R1 then.

12/R1 = 3

R1= 4

Let second resistance is R2 and current flowing through it is I_2

And third resistance is R3 and current flowing through it is I_3

If second and third resistor have equal value, then equal current will flow through them.

R2 = R3 and I2 = I3

I = I_1 + I_2 + I_3 = I_1 + 2I_2

6 = 3 + 2I_2

I_2 = 3/2

So R2 = V/I2 = 12/1.5 = 8 ohm

So R3 = R2 = 8 ohm