The blanks below can be filled in with three letters that differ only by the third letter to make three common words.

CH _ _ _ S
CH _ _ _ S
CH _ _ _ S

For example, if the first two letters were X's, the words would be

CHXXAS
CHXXBS
CHXXCS
------------ with the X's being the same two letters in all three words, and A, B, and C being three different letters.

HINT:
In two of the words, the CH makes the K sound.

Answers:

The letters are ORD, ORU, and ORE:

CHORDS
CHORUS
CHORES