In Harry Potter and the Deathly Hallows: Part 1, after destroying the locket Ron mentions that there are three more to go. But there were seven horcruxes. One of them, the book, was destroyed earlier in the chamber of secrets and one is this locket Ron destroyed. If there were three more to go, then what were the other two that were destroyed earlier? What Dumbeldore destroyed was fake, right?
Voldemort set out to split his soul in 7. And the last piece of soul remains within him, so in essence, there are 6 Horcruxes to be destroyed. From The Half-Blood Prince:
Riddle: Can you only split the soul once? For instance, isn't seven...
[The book explains this desire further: Seven is a magically powerful number.]
Of these Horcruxes, 2 were eliminated before Deathly Hallows:
- Harry destroyed the piece of soul in Riddle's diary in Chamber of Secrets;
- Dumbledore destroyed the piece of soul in Gaunt's ring in Half-Blood Prince.
This means that after Ron eliminated the locket Horcrux, only 3 were indeed left.
However... [Spoilers for the last movie below!]
What they and Voldemort don't know, and what Dumbledore (and later Snape) knew, is that Voldemort did create 7 Horcruxes: He created another Horcrux by mistake when he killed Harry's parents. A piece of soul was separated from his body and latched onto Harry, making Harry the last Horcrux.