This is because, to create a Horcrux, one most split their soul through the ultimate act of violence: murder. In doing so, they can leave part of their soul for safe (or not) keeping in a vessel separate from their body. The Dark Lord — aka Voldemort — created seven Horcruxes in an attempt to sustain immortality.
Voldemort only intended 7 soul pieces including his own soul piece, meaning 6 horcruxes then Voldy himself. Harry was the unintended 8th horcrux, due to his souls being so unstable by being split more than once.
So when Ron said three, it wasn't a mistake. He meant there were three more horcruxes to destroy before being able to destroy the fourth and final piece which was in fact Voldemort himself.
However, it should also be noted that Voldemort unintentionally split his soul into eight pieces when he had intended to render it into only seven. So what the true limit is on the number of Horcruxes one can create is not truly known.
In the movies, the Horcruxes are destroyed in these ways: The killing curse — “Avada Kedavra.” Fiendfyre — a curse that creates powerful and magical flames. Godric Gryffindor's sword — one that was pulled from the Sorting Hat.
Quirrell tried to put up feeble resistance on occasion, but Voldemort was far too strong for him. Quirrell is, in effect, turned into a temporary Horcrux by Voldemort. He is greatly depleted by the physical strain of fighting the far stronger, evil soul inside him.
Voldemort intentionally made six Horcruxes, but when he used Avada Kedavra on Harry, he unintentionally created a seventh Horcrux. Instead of dying, Lily's love for Harry created a counter 'curse' known as Sacrificial Protection and saved Harry.
So, Voldemort had to kill Snape for the wand's allegiance to switch. Voldemort knew he couldn't cast Avada Kedavra with the Elder Wand to kill its master, so he ordered Nagini to be the one who killed Snape. In reality, the Elder Wand belonged to Draco, as he was the one to disarm Dumbledore in the Astronomy Tower.
Nagini the Snake
Nagini was the last horcrux to be destroyed and also the last to be made. It was created when Peter Pettigrew bumped into Ministry of Magic employee Bertha Jorkins in Albania while Voldemort was still weak.
A Horcrux is considered destroyed when it's damaged beyond repair. In the case of living Horcruxes, “damaged beyond repair” would mean dead. So, yes, Avada Kedavra can destroy a Horcrux, as long as that Horcrux is a living being.
Scabbers was actually, as we know, Peter Pettigrew, an unregistered animagus hiding out as the Weasley's pet rat. No matter how hard he focussed his intention on turning the rat yellow, or even if he'd said the words rattus colovaria (erm, we did our best) the spell would never have worked because the rat wasn't a rat.
According to The Leaky Cauldron, when a fan asked if Hermione made sure that her parents could remember her again, Rowling said, "she brought them home straight away." This is good news, as it's awful to imagine Hermione's parents never remembering who she is or ever seeing her again.
Ron gets splinched when he apparates away from the Ministry after taking the Slytherin locket from Dolores Umbridge. Since Ron had not passed the test for apparition, it is possible that he may not have had sufficient practice to apparate.
However, Voldemort, after finding out that he taught at Hogwarts, took over his body, subsequently turning him into a temporary Horcrux, though Quirrell put up some feeble resistance. He died in vain while trying to murder Harry Potter in the Underground Chambers.
Harry drops the resurrection stone mainly because he did not need it anymore. He seeked closure in a way from the people who gave up their life protecting him and once he received it he did not need the stone anymore. Harry never wanted to be the master of death or bring them back.
Dumbledore travelled to Little Hangleton and found the remains of the shack, hidden amongst many weeds and brush. Dumbledore succeeded in passing through the enchantments protecting the shack and discovered the golden box holding the ring beneath the shack's floorboards.
Nonetheless, despite her initially benevolent nature, Nagini's blood curse eventually turned her into a snake permanently. Though it is unknown if she became evil as a result of that, or was turned evil as a result of Voldemort turning her into a Horcrux. As a snake, Nagini was completely loyal to Lord Voldemort.
It wasn't just inanimate objects that could become Horcruxes. Lord Voldemort's beloved snake companion, Nagini, was also transformed into one, and Dumbledore seemed to know it, ominously predicting that a day would come where Voldemort would 'fear for his snake'.
Though it may later be revealed that the Dark Lord did indeed know that Nagini was a Maledictus, there's currently no indication that he (or anyone else) ever thought that she was anything other than a snake who was made into one of the Dark Lord's Horcruxes.
He was on the point of collapse when it happened,” Rowling said. “Dumbledore didn't want to lose his wand at that point and Draco disarmed him. So that meant that the wand gave Draco its allegiance, even though Draco never knew it, even though Draco never touched it.
Defeating The Dark Lord with his signature spell
Disarming is about defence, not murder. Voldemort was a killer, Harry was not. It was an important distinction for Harry, who had learned so much about Voldemort and seen first-hand the ways in which they were similar.
Draco Malfoy's mother Narcissa was cold, cunning and devoted to the Dark Lord. But she was also a mother, which meant she was willing to risk everything to make sure her son was safe. When Harry survived Voldemort's Killing Curse for the second time, Narcissa pretended he was dead so she could get to Draco.
While most fans agree that Harry is the unrefuted Chosen One, the final Deathly Hallows film (in particular) sways heavily toward highlighting a heroic Neville, revitalizing the debate regarding which boy is better suited for the iconic title. "The one with the power to vanquish the Dark Lord approaches ...
Though Harry encountered many opportunities that deemed Avada Kedavra necessary, it remains as one of the Unforgivable Curses Harry Potter never cast. For one, he viewed the spell as an immoral practice commonly used by users of the Dark Arts.