triennial(n = noun.time) tercentenary, tercentennial - the 300th anniversary (or the celebration of it);
is a kind of anniversary, day of remembrance

   triennial(s = adj.all) - occurring every third year or lasting 3 years;

1.  Continuing three years; as, triennial parliaments; a triennial reign. Howell. [1913 Webster]

2.  Happening, coming about, or appearing once in every three years; as, triennial elections; a triennial catalogue; a triennial visitation. T. Warton. [1913 Webster]

   Something which takes place or appears once in three years. [1913 Webster]

triennial, adj. & n.
--adj.
1 lasting three years.
2 recurring every three years.
--n. a visitation of an Anglican diocese by its bishop every three years.

Derivative:
triennially adv.

Etymology:
LL triennis (as TRI-, L annus year)

N  regularity of recurrence, periodicity, intermittence, beat, oscillation, pulse, pulsation, rhythm, alternation, alternateness, alternativeness, alternity, bout, round, revolution, rotation, turn, say, anniversary, jubilee, centenary, catamenia, courses, menses, menstrual flux, rota, cycle, period, stated time, routine, days of the week, Sunday, Monday, months of the year, January, feast, fast, Christmas, Easter, New Year's day, Allhallows, Allhallowmas, All Saints' Day, All Souls', All Souls' Day, Ash Wednesday, bicentennial, birthday, bissextile, Candlemas, Dewali, groundhog day, Halloween, Hallowmas, Lady day, leap year, Midsummer day, Muharram, woodchuck day, St. Swithin's day, natal day, yearbook, yuletide, punctuality, regularity, steadiness, periodic, periodical, serial, recurrent, cyclical, rhythmical, recurring, intermittent, remittent, alternate, every other, hourly, diurnal, daily, quotidian, tertian, weekly, hebdomadal, hebdomadary, biweekly, fortnightly, bimonthly, catamenial, monthly, menstrual, yearly, annual, biennial, triennial, centennial, secular, paschal, lenten, regular, steady, punctual, regular as clockwork, periodically, at regular intervals, at stated times, at fixed established, at established periods, punctually, de die in diem, from day to day, day by day, by turns, in turn, in rotation, alternately, every other day, off and on, ride and tie, round and round.

N  vegetable, vegetable kingdom, flora, verdure, plant, tree, shrub, bush, creeper, herb, herbage, grass, annual, perennial, biennial, triennial, exotic, timber, forest, wood, woodlands, timberland, hurst, frith, holt, weald, park, chase, greenwood, brake, grove, copse, coppice, bocage, tope, clump of trees, thicket, spinet, spinney, underwood, brushwood, scrub, boscage, bosk, ceja, chaparal, motte, arboretum, bush, jungle, prairie, heath, heather, fern, bracken, furze, gorse, whin, grass, turf, pasture, pasturage, turbary, sedge, rush, weed, fungus, mushroom, toadstool, lichen, moss, conferva, mold, growth, alfalfa, alfilaria, banyan, blow, blowth, floret, petiole, pin grass, timothy, yam, yew, zinnia, foliage, branch, bough, ramage, stem, tigella, spray, leaf, flower, blossom, bine, flowering plant, timber tree, fruit tree, pulse, legume, vegetable, vegetal, vegetive, vegitous, herbaceous, herbal, botanic, sylvan, silvan, arborary, arboreous, arborescent, arborical, woody, grassy, verdant, verdurous, floral, mossy, lignous, ligneous, wooden, leguminous, vosky, cespitose, turf-like, turfy, endogenous, exogenous, green-robed senators of mighty woods, this is the forest primeval.