So this is a specific example but here we go:
Say you have a Salamence, Aurorus, Lanturn, and Trevenant.
First turn, the Lanturn uses Soak on the Salamence and the Trevenant uses Forest's Curse.
On the second turn, Aurorus uses Freeze-Dry on the Salamence.
Would the damage multiplier be 2, 3, or something else?