how to delete unnecessary character?

I cannot describe it in English.

e.g, categories are TITLE

001 test
blahblahblah.mp3
002 test hblahblahbla.mp3
...
123 test lahblahblahb.mp3

Bolded are what will delete, how to made it with regular expressions?

THX!

new action

regular expression

_FILENAME

(\d*) (.) (.)

$3