POP __ QUESTION is a phrase used to describe a question that is asked suddenly and unexpectedly, usually in the form of a surprise. It is often used to test someone's knowledge or to catch them off guard. The three letters in the crossword solution are 'QUI', which is short for the Latin word 'QUID' meaning 'what'. Therefore, the full phrase is 'Pop Qui Question', which can be interpreted as 'What is the sudden question?'.