3 12 21 30

The given series of numbers is 3, 12, 21, 30.

To understand the pattern in this series, we can try to calculate the differences between consecutive terms. Let's calculate the differences:

12 - 3 = 9
21 - 12 = 9
30 - 21 = 9

The differences between consecutive terms are all equal to 9. This indicates that the series is an arithmetic progression, where each term is obtained by adding a fixed constant value to the previous term.

To find the next term in this arithmetic progression, we can add the common difference of 9 to the last term of the series. In this case, the last term is 30.

30 + 9 = 39

Therefore, the next term in the series is 39.