INSERT - A - WORD III

The same three-letter word can be inserted into the three words below to make new words. What is the three-letter word, and what are the new words?

PIE
SLY
WING
BLING

HINT:
The new words make from PIE and SLY will rhyme, as will the new words make from WING and BLING.

Answer:

The three-letter word is INK:

PINKIE
SLINKY
WINKING
BLINKING

Credit:
Larry Shearer
INVISIBLE INK