You need a macro that supports regular expressions for file renaming. See Rename Files Mystery for a couple alternatives.
Then you need a regular expression to plug in to it:
Search for : ^(.{3}).+?.
Replace with: $1_EXT.
That gets you:
- bkr-hhdjj2.jpg >> bkr_EXT.jpg
- alp-jhhf45.jpg >> alp_EXT.jpg