The term 'not certain' can be expressed in three different ways in a crossword puzzle: 'UNSURE' (6 letters), 'IFFY' (4 letters), and 'indoubt' (7 letters). 'UNSURE' is the most common way to express uncertainty, while 'IFFY' is a slang term that implies something is uncertain or unreliable. 'Indoubt' is a less common way to express uncertainty, but it still conveys the same meaning. All three terms are used to describe a situation where the outcome is not certain.