Can you help me choosing the right tools for this project? It's a 3d browser based multiplayer online fighting game. A few 3d functions are necessary for positioning, but no heavy rendering will happen. It needs very low ping rates, though, as it will be very dynamic. HTML5 + three.js + some networking lib seems like a fair choice, but I'm concerned about performance and cross-browserism. It's for learning purposes but I can use it for professional showoff. Thoughts?
Ping rates aren't dependent on the library you use at all. They're going to be dependent on your network. I know you probably just meant that you want low latency, but it helps to get the terms right. :)
If you don't need IE support right now, I would lean towards WebSocket. It does what you need for an MMO just fine. http://www.websocket.org/
If you need IE support, there are wrapper libraries that emulate WebSocket using Flash. https://github.com/gimite/web-socket-js
You can also use the HTML5Labs IE prototype for WebSockets if you don't need any "regular" users on IE to run your game today. http://html5labs.interoperabilitybridges.com/prototypes/websockets/websockets/info
For the networking library, I would look at Socket.IO. It uses WebSockets (when possible) but, as seanmiddleditch mentioned, falls back on Flash (or something) for IE. Your server will use Node.js if you choose to use Socket.IO, which is a great choice in my opinion. For hosting, you can refer to the Stack Overflow question "Where can I host a Node.js app?"
As for graphics on the client, three.js looks awesome and it's on my todo list to play with, so I agree with that choice.