Two series alternate here, with every third number following a different pattern. In the main series, 3 is added to each number to arrive at the next. In the alternating series, 5 is subtracted from each number to arrive at the next.

This is a simple subtraction series; each number is 4 less than the previous number.

This is a simple multiplication series. Each number is 2 times greater than the previous number.

In this series, the letters progress by 1; the numbers decrease by 3.