DEAD LETTER DEPT.
GAMES Big Book of Games
Mike Shenk

The same three letters, in the same order, will complete all four of these common English words.

CA _ _ _ ZE
GY _ _ _ ES
LO _ _ _ DED
TI _ _ _ NESS

HINT:
The three letters, read as a single word, name a letter of the Greek alphabet.

Answers:

The letters are PSI:

CAPSIZE
GYPSIES
LOPSIDED
TIPSINESS