For me, I think Piper (and Leo) not believing Paige about her suspicions on Cole is probably due to how far they've come to trust Cole, even before Paige came into the picture. They had a whole history with him that ended on "he's done a lot for us, he was there for us when Prue died, he's good now". Cole, unintentionally, successfully infiltrated the Halliwell family the way Balthazor was supposed to. Unfortunately for Cole/the Source, Paige wasn't there so all she heard was "he used to be a demon" and her Spidey senses went up. Phoebe is a mixture of love blindness and whatever mojo the Source and the Seer are putting her under. For now, Paige is alone on this. I also think Phoebe knew that Paige disliked and distrusted Cole but she didn't know Paige actively HATED Cole. I also think she's naive enough to believe all of Paige's negative feelings about Cole would disappear after Cole and Phoebe got married. Either way, I'm glad we have gotten to the Smart Paige era. Can't wait for more!