tril ‧ o ‧ gy /ˈtrɪlədʒi/ BrE AmE noun ( plural trilogies ) [countable]
[ Date: 1600-1700 ; Language: Greek ; Origin: trilogia 'group of three plays' , from tri- 'three' + -logia '-logy' ]
a series of three plays, books etc that are about the same people or subject:
part 2 of a trilogy