The same three letters, in the same order, can fill the blanks below to create new words.

LO _ _ _G
RI _ _ _ G
CLO _ _ _ G
_ _ _ KING
_ _ _ K

Answers:

The missing letters are SIN:

LOSING
RISING
CLOSING
SINKING
SINK

Credit: LA Times Crossword
Sylvia Bursztyn and Barry Tunick
SANS SIN