Tri-Hex is a mill game that was invented by T. H. O’Beirne in 1962. It was first described in an article in New Scientist magazine, and was derived by exploring alternate board configurations for Tic-Tac-Toe.a
Each player has four pieces, which are first (the placement phase) placed one at a time on the board. Once all the pieces are on the board, players take turns moving a single piece along a line to the (single) vacant location. The first player to form a row of three along a line wins the game (this can be during either of the placement or movement phases).
O’Beirne provides analysis of the play in the original article (below in References).
O’Beirne, T. H. (). ‘Puzzles and paradoxes 54: New boards for old games’. New Scientist (269): pages 98–99.