The Collaborative International Dictionary of English v.0.48:

Tercentenary \Ter*cen"te*na*ry\, a. [L. ter thirce + E.
   centenary.]
   Including, or relating to, an interval of three hundred
   years. -- n. The three hundredth anniversary of any event;
   also, a celebration of such an anniversary.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tercentenary
    n 1: the 300th anniversary (or the celebration of it) [syn:
         tercentennial, tercentenary, triennial]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "tercentenary":
   C, anniversary, annual holiday, bicentenary, bicentennial,
   biennial, birthday, bissextile day, cental, centare, centenarian,
   centenary, centennial, centennium, centigram, centimeter,
   centipede, centistere, centred, centref, centrev, centumvir,
   centumvirate, centurion, century, commemoration, cwt, decennial,
   diamond jubilee, golden wedding anniversary, gross, hecatomb,
   holy days, hundred, hundredweight, immovable feast, jubilee,
   leap year, long hundred, name day, natal day, octennial, one C,
   quadrennial, quasquicentennial, quincentenary, quincentennial,
   quinquennial, septennial, sesquicentenary, sesquicentennial,
   sextennial, silver wedding anniversary, tercentennial, tricennial,
   triennial, wedding anniversary