S, O and N. They are the first letters of the months of the year.
d-u-m-b-a-s-*-!-!-!
Check your list. Are you sure it's not m a m j j a s o? Because in that case the next letter has to be n. N for November.
The letter O for the month of October. March, april, may, june, july, august, september...
N for november. The sequence starts with march.