Ship's distress call (3)

I believe the answer is:
sos
(Other definitions for sos that I've seen before include "Help with dots and dashes" , "Mayday call" , "Appeal for help" , "Help code in 13 Across" , "Coded distress signal" .)
I believe the answer is:
sos
(Other definitions for sos that I've seen before include "Help with dots and dashes" , "Mayday call" , "Appeal for help" , "Help code in 13 Across" , "Coded distress signal" .)