The glossary about multiplication says
When asked to apply more than one multiplier to a roll, multipliers do not multiply each other. Instead, combine them into a single multiplier, and each additional multiple adds 1 less than its value to the first multiple. For example, if you are asked to apply a multiplier × 2 twice, the result would be × 3, not × 4.
The critical hit multiplier of a halberd is × 3 and deals damage of × 2 when preparing for a charge, so a critical hit when dealing with a charge inflicts × 4 damage.