We know that in the unique Nash equilibrium each firm's output is 30.
In a long run competitive equilibrium the price is equal to the minimum average cost, which is 30. Thus the total output of the firms is Q such that 30 = 120 Q, or Q = 90.
A monopolist chooses the output for which MC is equal to MR. We have TR(y) = y(120 y), so MR(y) = 120 2y. Thus the monopolist's output y satisfies MC(y) = 30 = 120 2y, or y = 45.
In summary, the three outcomes are given in the following table.