PEACE PIECE
Credit: LA Times Crossword
Sylvia Bursztyn and Barry Tunick

The same three letters, in the same order, will change each of the following words into new ones.

1. AD
2. BEE
3. FORD
4. DEN
5. RANT

HINT:
The three letters are inserted into words 1, 2, and 3, and precede words 4 and 5.

Answer:

The letters are WAR:

AWARD
BEWARE
FORWARD
WARDEN
WARRANT