Step 1:
Meghanāda (Indrajit) and Akshayakumara are indeed described as sons of Ravana in Ramayana.
Thus, Assertion (A) is correct.
Step 2:
Sahitya Darpana was authored by Vishvanatha Kaviraja, not Mammata.
Hence, Reason (R) is incorrect.
Step 3:
Mammata is famous for the work:
\[
\boxed{\text{Kāvyaprakāśa}}
\]
Step 4:
Therefore:
\[
\boxed{\text{A is correct but (R) is not correct}}
\]
Hence, the correct answer is:
\[
\boxed{\text{(3)}}
\]