The multiplier will help you accumulate points for the event. It will also help you reach the next milestone faster, in order to claim better rewards.
To increase your multiplier, you need to win a minimum of two out of three battles in a 3 vs 3 arena. Losing the arena match will reset your multiplier.
Base multiplier: 1x
First win: 1.25x
Second win: 1.5x
Third win: 2x
Fourth win: 2.5x
Fifth win: 3x
You can check your current multiplier at the beginning of the series match.