Douglas Harper's Etymology Dictionary
Wiktionary
triannual
a. Happening three times each year.
Wikipedia
Triannual
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Usage examples of "triannual".
That would put us back on the triannual schedule I've been hoping to maintain all along.
As I said in my preface to that issue, I'm hoping to be able to maintain a triannual publication schedule for the magazine.
As I said in my preface to that issue, I’m hoping to be able to maintain a triannual publication schedule for the magazine.