Prue is dead and, therefore, technically, so is The Power of Three as there are now only two sisters left.
What Piper and Phoebe are unaware of is that they have a long-lost half sister about whom they know nothing. She is the product of Patty's affair with her Whitelighter, Sam, and, in the Halliwell tradition, her name starts with a "P" as she is called Paige.
I found this episode really weak for a season opener and thought the writers could have done much better. Naturally, for the show to continue, they had to bring in another sister, but the whole thing was quite cheesy. The only reason I gave it a 7 was because Alyssa Milano and Holly Marie Combs did an excellent job of portraying their character's immense grief. The rest of it was pretty average.