User blog comment:Kacieh/Harry Potter and the Deathly Hallows- Part 1 Review Page/@comment-3210023-20101229184153/@comment-148.83.132.110-20110104125719

The movie is correct; they only know of three. They think Voldemort split his soul into seven pieces - ie. six horcruxes and one piece of soul in his own body. They had already destroyed the diary, the ring and the locket, which leaves three that they knew of - the diadem, the cup and the snake. They didn't know that Voldemort had accidentally created a seventh horcrux, Harry - splitting his soul in eight.