The Collaborative International Dictionary of English v.0.48:

Tricentenary \Tri*cen"te*na*ry\, a. [Pref. tri- + centenary.]
   Including, or relating to, the interval of three hundred
   years; tercentenary. -- n. A period of three centuries, or
   three hundred years, also, the three-hundredth anniversary of
   any event; a tercentenary.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tricentenary
    adj 1: of or relating to or completing a period of 300 years
           [syn: tricentenary, tricentennial]