triannual

As an adjective triannual

is happening three times each year.

As a noun triennia is

.

As adjectives the difference between triannual and false

is that triannual is happening three times each year while false is (label) one of two states of a boolean variable; logic.

As adjectives the difference between annual and triannual

is that annual is happening once every year while triannual is happening three times each year.

As a noun annual

is an ; a book, periodical, journal, report, comic book, yearbook, etc, which is published serially once a year, which may or may not be in addition to regular weekly or monthly publication.