Triennial
TRIEN'NIAL, adjective [Latin triennis, triennium; tres, three, and annus, year.]
1. Continuing three years; as triennial parliaments.
2. Happening every three years; as triennial elections. triennial elections and parliaments were established in England in 1695; but these were discontinued in 1717, and septennial elections and parliaments were adopted, which still continue.