Finding the Right One-Word Substitute

The question asks for a single word that can replace the phrase "Something happening in three years". This means we are looking for a word describing an event or occurrence that takes place every three years.

Analyzing the Options for Frequency

Let's look at the provided options and understand what frequency each word indicates:

• Annual: This word is used for something that happens or is done once every year. Think of an annual report or an annual festival.
• Triennial: This word describes something that occurs every three years. The prefix 'tri-' relates to three, and '-ennial' relates to years.
• Biennial: This term refers to something that happens every two years. The prefix 'bi-' indicates two.
• Millennial: This word relates to a millennium, which is a period of one thousand years. It can also refer to a person born in the 1980s or 1990s. This word does not describe an event happening every three years.

Based on the analysis, the word that specifically means "happening in three years" or "occurring every three years" is Triennial.

Therefore, the correct one-word substitute for "Something happening in three years" is Triennial.

Revision Table: One-Word Substitutes for Frequency

Additional Information: Understanding -ennial Words

Many words describing events by the number of years they occur use the suffix '-ennial'. Understanding the common prefixes can help you remember their meanings:

• 'annus' (Latin) means year, hence Annual (every year).
• 'bi-' means two, hence Biennial (every two years).
• 'tri-' means three, hence Triennial (every three years).
• 'quadr-' means four, hence Quadrennial (every four years, like the Olympic Games).
• 'dec-' means ten, hence Decennial (every ten years).
• 'cent-' means hundred, hence Centennial (every hundred years).
• 'mill-' means thousand, hence Millennial (relating to a thousand years).

These words are useful in clearly stating the frequency of recurring events or periods.