Today at my job, (i work at a maintenance and die shop) i got to take apart a 3in chuck, because it got grinding dust packed in the hobs.
I have never taken apart a 3 jaw chuck before and it is very interesting to see how it works, it has 3 little knobs that the chuck key goes into and advances a tapered spiral that closes the jaws.
I’ve taken ours apart twice, once to clean it, the second because I put it together wrong after cleaning it. Still pretty neat design.
4 jaws are less interesting, but neat nonetheless.