Three and Eight have spent most of the movie at each others' throats. When Three wants someone to help him reenact a knife attack, Eight goes along with it because he's the only one standing up. Given their previous interactions, as soon as Three raises the knife, everybody else in the room jumps up thinking Eight is about to get stabbed. Three and Eight, for their part, are relatively cool-headed about it.
The Oh, Crap expression Juror #3 has when he accidentally contradicts his own statement, asking how the old man (whose testimony he's been using as evidence) could be sure about anything. Especially funny if you see a staged version, as some productions will have #4 give him a "You idiot" sort of look while everyone else just stares at him.