1. [n]  the 300th anniversary
2. [adj]  occurring every third year or lasting 3 years

TRIENNIAL is a 9 letter word that starts with T.

1. \Tri*en"ni*al\, a. [L. triennium the space of three
   years; tri- (see {Tri-}) + annus year. See {Annual}.]
   1. Continuing three years; as, triennial parliaments; a
      triennial reign. --Howell.
   
   2. Happening, coming about, or appearing once in every three
      years; as, triennial elections; a triennial catalogue; a
      triennial visitation. --T. Warton.
   
   

2. \Tri*en"ni*al\, n.
   Something which takes place or appears once in three years.