Step 1:
According to mythology, fourteen Ratnas emerged during Samudramanthana.
Thus, Assertion (A) is correct.
Step 2:
Kaustubha Mani was received by Lord Vishnu, not Shiva.
Hence, Reason (R) is incorrect.
Step 3:
Samudramanthana is described in several Puranic texts.
Step 4:
Therefore:
\[
\boxed{\text{A is correct but (R) is not correct}}
\]
Hence, the correct answer is:
\[
\boxed{\text{(3)}}
\]