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.
1. Existing or lasting for two years; changed every
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
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,
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.