In the question above they are asking about the language accepted by the grammar not about the grammar,
we can rewrite the grammar as,
S -> ( B )
B-> 1B | 2B | 3B
B-> 1 | 2 | 3
you can generate any combination of 1,2,3 by the above grammar prefixed by '(' and suffixed by ')'. For this language, Finite automata can also be constructed.
let q0 be the initial state and q3 be the final state with transition function below->
(->q0, '(' ) = q1
(q1, 1) = q2
(q1, 2) = q2
(q1, 3) = q2
(q2, 1) = q2
(q2, 2) = q2
(q2, 3) = q2
(q2, ')' ) = q3*
The language for which Finite automata can be constructed is regular. Hence, the language generated by this grammar is regular.