Something that happens every three years is a triennialevent.
Something that happens every six years is sexennial.
Biennial or biannual can refer to something which happens every two years.
the word is novennial....
Semicentennial
If something happens twice a month it is called biweekly because it happens every other week and a month has 4 weeks. there is also bimonthly which is every other month but not really a direct word for something that happens twice a month.
Something that happens every three years is a triennialevent.
Something that happens every six years is sexennial.
Quarter. At every Quarter of the financial year (every 3 months).
Biennial or biannual can refer to something which happens every two 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.
Annual
The definition of the word quotidian is something that happens every single day. A good sentence is, he has to deal with the quotidian traffic on his way to work again.
The word you are looking for is "cause."
The term for something that happens by itself is "spontaneous."
Adverb.
The word is triennial.