The same three-letter word, when properly placed, will change each word below into a new one.

TRY
SING
BLING
DEFER
DESCANT

Answer:

The three-letter word is END.

trENDing
sENDing
blENDing
defENDer
descENDant

Credit: NY Times Crossword - Aug. 24, 1986
Arnold Moss
NO END IN SIGHT  (Idea)