Depending on how confident Pansy, Theo, and Blaise appear to others outside the castle, they could help solidify his story. Yes, Draco's working on it - whatever it is. He's working hard. Tirelessly, in fact. But if they haven't been visibly involved up to this point, suddenly involving them might look odd. It feels strange, but now Draco hopes at least one of them had been asked to keep an eye on him all year. Laying a false trail will be that much easier.