(3).

There are no points at which f " (x) = 0, but at x = ± 2 the function f is not continuous, so test for concavity in the intervals (- ∞, - 2), (- 2, 2) and (2, ∞) as shown in the table.

Test intervals    x - Value       Sign of f " (x)           Conclusion

(- ∞, - 2)                x = - 3             f " (- 3) > 0           Concave upward

(- 2, 2)                   x = 0               f " (0) < 0              Concave downward

(2, ∞)                    x = 3                f " (3) > 0              Concave upward