Next number in sequence 3 - 6 12 4 20?

-6 - 3 = - 9

12 - (-6) = 18

4 - 12 = - 8

20 - 4 = 16

The first difference is - 9. The third difference is - 8.

Maybe the fifth difference is - 7.

That makes the next term 13.

3, - 6, 12, 4, 20, 13