Multiplying More Than Once
When you are asked to multiply a value or roll more than once, the multipliers (×2, ×3, and so on) are not multiplied by one another. Instead, you combine them into a single multiplier, with each extra multiple adding 1 less than its value to the first multiple. For example, if you apply a ×2 multiplier twice, the result is equivalent to multiplying the value by 3 (or rolling the damage three times), not multiplying it by 4.
Source
Core Rulebook pg. 242
