The word is triennial.
A triennial event happens every three years.
Something that happens every three years is a triennialevent.
tercentennial
The phrase "once every 3 years" can be expressed as "triennial." This term refers to an event or occurrence that happens every three years. Alternatively, you could also say "every three years" for clarity.
It is a triennial event. A triennial event happens three times a year.
Every person is different-but most likely you will be out a lot of money
Something that happens every six years is sexennial.
Something that happens once a century.
Quadrennial
As far as I know, nothing special happens every 100,000 years. A full precession cycle takes about 26,000 years.
An event that occurs every thirty years is called a "triennial" event. However, this term can be misleading since "triennial" technically refers to something happening every three years. For events specifically occurring every thirty years, the term "tricenary" can be used, although it is less common.
Every three years