Can you recommend a game dev environment that is 3D, cross-platform, and (ideally) open-source, while being actually fun to code in?

Defining fun: I love coding in Ruby and Lua. Learning new languages is fun - it might be neat to do a game in Erlang or something. But I hate coding in C++ and am getting pretty tired of Java.

My girlfriend and I started coding a game together as a hobby. It may never see the light of day - we're primarily just trying to have a good time. To that end, we'd like to do it in a fun language.

So we started it in Lua, using the Löve framework, which has been awesome. Unfortunately ... it looks like we're definitely going to need a 3D environment, with proper 3D acceleration. As soon as I started doing a little vector math for lighting some simple 3D meshes, poor Lua was brought to a crawl. We have a very simple demo of structures floating in space ... and we only get 15fps out of it. Ouch.

I've been looking at 3D game environments, but the thought of having to go back to malloc() just makes me sad. Any thoughts?