Juror #3's Dramatic Change (Spoilers)

For Lee J there was no single piece of evidence that swayed him. But the reasonable doubt was mounting and he was left between a rock (Lee J's unsupportable belief in his guilt) and a hard place (reasonable doubt).

Every third person who complains will be shot. Two people have complained already!

For me, juror 3's response has to be connected to the photo. There is emphasis on it when he throws his notepad and is angrily ripping it up and I think in that split/second he considered what if his son was in the defendant's position? 3 finally understand the gravity of the boy's life being at stake and his emotions beat the flawed evidence. It would make sense if his son had faced trouble in the past as well since he mentions his son punching him in the jaw. Literally watched this film an hour ago, it holds up so brilliantly what an amazing movie!!

The 1997 version and the revised playscript add some extra dialogue that explains this very poignantly.

Juror 3: The kid said "I'm going to kill you" and he did! To HIS OWN FATHER! I don't care what kind of man he was--THAT WAS HIS FATHER! Jesus...why I am I the only one who sees? (Breaking down in tears)...I can feel the knife going in...

Juror 8: He's not your boy. He's someone else.

Juror 4 (putting his hand on 3's arm): ...Let him live.

Juror 3: ...All right. Not guilty.

So it's obvious he's come to realize that his own viciousness against the defendant is rooted in his own problems with his son--whom, in his view, he did everything for, but who "stabbed him in the heart" by breaking off contact with him.