Corrected entry: Phoebe says Piper and Prue were able to defeat Shax without the power of three and Piper's response is it was because "Prue was the strongest." Piper would have no recollection of this event as Tempus reversed time bringing Piper back from the dead in the timeline that she was shot in. The time reversal affected everyone's memory except Phoebe, Leo and Cole who were in the Underworld. (00:14:30)
Correction: Also Phoebe was in the underworld and was not effected by Tempus' change, and Phoebe could have told her what happened. I assume they all know what happened before the time change at this point.
Corrected entry: Paige apparently only gets her witch powers in this episode, but that doesn't fit with what we know of magic from the rest of the series. If Paige was born a witch, she would have always had her witch powers unless they were bound (which we know they weren't since she would have also lost her orbing ability until now), so she should have been telekinetically orbing things her whole life.
Correction: I believe that Piper simply meant that statement in general and not because she actually remembered the event. She also doesn't say anything else about the timeline that was altered. In Piper's mind, Prue had always been the strongest of the three of them, because she was also the oldest. Piper and Phoebe use that statement several times throughout the series.