GOT MILK
Harvey Estes

The same three letters, in the same, consecutive order, can be inserted somewhere into each word below to make  new, unrelated words.

ABS
SINS
MUMS
CALLS
MASSES

Answer:

The letters are USE:

ABUSES
SINUSES
MUSEUMS
CALLUSES
MASSEUSES

Explanation of puzzle title:
To use - as to take advantage - milk