1) Occurring every third year 2) Period 3) Period of time 4) The 300th anniversary 5) Time period 1) Tercentenary 2) Tercentennial 
• (a.) Continuing three years; as, triennial parliaments; a triennial reign. • (n.) Something which takes place or appears once in three years. • (a.) Happening, coming about, or appearing once in every three years; as, triennial elections; a triennial catalogue; a triennial visitation.
[adj] - occurring every third year or lasting 3 years
triennial 1. Existing or lasting for three years, three-years; changed every three years. 2. Recurring every three years. 3. A commemoration or celebration of an event that occurred three years earlier; a third anniversary.
adjective occurring every third year or lasting 3 years