Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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