I don’t know if it has an official name, but it is commonly called the “dun dun dun” sound effect, sometimes with an alternate spelling such as “dun dun duuuuunnnnn” or with similar emphasis on the last note.
Googling “dun dun dun” will give you lots of hits. If there’s an official name for it (other than dun dun dun) you can probably find it that way.