12 is divided into 6 and 6 fields.
6 into 4, 3+3 games (6 into 3 played 3 games, the remaining 3 played 3 games, A played 1 game with B and C respectively, ***2 games, B and C played 1 game, integral, if the game is tied, the winner will be 6 into 4).
Semifinal: 4 in 2 2 games+1 game (third place, third place)
Finals: 2 games 1 1.
I wish you happiness.