Earlier quoted context omitted.
All of those are WebKit-based, which was Chromium's rendering engine until recently. The rest of Chromium is primarily UI and network code, along with their sandboxing system and extensions - WebKit's "ports" have their own implementation of said network code tying into the framework that they're porting to (GTK+ or Qt, usually), and all three browsers you list use the WebKitGTK+ port. uzbl, the one I've used the mos…
Do you know of any independent rendering engines? That's something I'd truly be interested in seeing. It seems like a herculean task to undertake. Edit: Thanks very much for the links. This will be a lot of fun testing out. Just responding here to avoid triggering the anti-flamewar protection.
It wasn't easy ten years ago, and a modern browser is at least an order of magnitude more complicated (WebGL, more complicated CSS, JS support, heck I'm not even really counting JS JIT here...).