Necro Bump! Should have checked the date =/
Criticals are separate from the rest of the weapon damage calculation and is calculated last and as its OWN multiplier. I've never read anywhere that it has to follow the x2 x3 x4 x5 etc rule. Everything before that part of the damage calculation follows that rule, IE Two doublings is a tripling. If they made crits follow this rule Scythes would be virtually worthless with its x4 Multiplier.
Depending on your GM it can be handled several ways.
1d8 + Str + other static damage enhancements like power attack, enhancement bonus, Smite Evil, feats etc
Triple that for Charging to get
3(1d8 + Damage Enhancements)
Critical that to get 3(3(1d8 + Damage Enhancements))
Another way would be to treat that triple damage as its own dice roll
So instead of 3(1d8 + Damage) you're getting 3d8 + 3(Damage). This method means you're less likely to be boned by a low dice roll.
Then triple that for the crit for 3(3d8 + 3(Damage)) = Total damage dealt
The third and final way Ive seen this resolved is just treat everything like it was an individual dice roll/damage roll. 9d8 + 9(Damage)