Hi Graham,

This is not a silly question at all. I did some searching and I don't have a good answer for you.

My university subscribes to the online version of the Oxford English Dictionary so I went there and looked up triannual. The response was:

    Occurring every three years; lasting for three years; = TRIENNIAL.

1640 Par. Acc. St. Barth. by Exchange in Archæologia XLV. 78 Pd. the ringers for joy of the tryannual Parliament, o. 2. 6. 1656 EARL OF MONMOUTH tr. Boccalini's Advts. fr. Parnass. I. lxix. 133 He was deputed a Triannual President of the Isle of Negrapont.

 ¶ b. Occurring thrice a year.

1901 Daily Chron. 8 June 5/2 The ladies..we learn..will hold ‘tri-annual invitation meetings’... That, we suppose, means an invitation once in four months.

I also looked up triennial it it has only the first meaning.

I tried biannual and got:

    A. adj. Used as = Half-yearly.    B. n. = BIENNIAL n. Hence biannually adv.

1877 OUIDA Puck xii. 123 Every half-year his lawyers transmitted him..the biannual rental. 1884 Illustr. Sydney News 26 Aug. 15/1 Plant out..annuals and bi-annuals. 1882 Century Mag. XXIII. 647 A change in the fashion of her clothes bi-annually at least.

and biennial:

A. adj.
     1. Existing or lasting for two years; changed every two years.

1621 HOWELL Lett. I. I. xli, The Duke is there [at Genoa] but Biennial, being chang'd every two years. a1711 KEN Hymnoth. Poet. Wks. 1721 III. 12 Biennial Stores they [ants] treasure under Earth. 1854 WOODWARD Mollusca (1856), The land-snails are mostly biennial.

b. esp. of plants; see B.

1691 RAY Creation I. (J.), Some..very long lived, others only annual or biennial. 1755 Gentl. Mag. XXV. 69 The common hemlock is biennial. 1805 KNIGHT in Phil. Trans. XCV. 262 Annual and biennial plants.

2. Recurring, happening, or taking place once in every two years.

1750 JOHNSON Rambl. No. 61 6 Whom he condescends to honour with a biennial visit.

B. n. Bot. A plant which springs from seed and vegetates one year (or growing season), and flowers, fructifies, and perishes the next.

1770 WARING in Phil. Trans. LXI. 385 Biennials..are the natural..product of such places. 1815 Encycl. Brit. (ed. 5) III. 610 Of the esculent kinds, the cabbage, savoy, carrot, parsnip, beet, onion, leek, etc., are biennials.

Searches in other dictionaries resulted in much the same confusion. Triannual does have the meaning you want, at least according to the Oxford English Dictionary, but it also means every three years. I think you might have to stick with three times a year or thrice yearly.

Harley

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