Explanation
The pattern in the series alternates between 3 and a number that increases by 3:
3 | 3 | 3 | 6 | 3 | 9 | 3 | ?
The numbers in the positions 2, 4, and 6 (i.e., the non-3 positions) increase by 3:
3 → 6 → 9.
Following this, the next non-3 number would logically be 12.
Thus, the completed series is:
3 | 3 | 3 | 6 | 3 | 9 | 3 | 12.
