The Solution:
Let the 3 girls be represented with A, B, and C.
At round 1:
Let C be the loser while A and B are winners.
A has 12 chips
B has 21 chips
C has 39 chips
At round 2:
Let B be the loser while A and C are winners.
A has 24 chips
B has 42 chips
C has 6 chips
At round 3:
Let A be the loser while A and C are winners.
A has 24 chips.
B has 24 chips.
C has 24 chips.
Therefore, the girl A has 12 chips, girl B has 21 chips and girl C has 39 chips at the beginning of the game.