The partial words below can be completed by adding the same three letters, in the same order, to each of them. What are the letters and what are the new words?

GU _ _ _
AG _ _ _
EX _ _ _
ST _ _ _
AN _ _ _

HINT:
WH _ _ _ could be added to the list.

Answers:

The letters to be added are ILE:

GUILE
AGILE
EXILE
STILE
ANILE 
(WHILE)