The word is triennial.
A triennial event happens every three years.
Something that happens every three years is a triennialevent.
tercentennial
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.
Every three years
Quinquennial = something that happens every 5 years. Lustral = also something that happens every 5 years or we could invent: semi-Decennial: where semi is half and Deccennial happens every 10 years.
This is a blue moon. It happens very rarely, once every two to three years or so.