Just to add my 2 cents:
If you're planning to use OpenGL, then your options are even wider because many OpenGL apps such as games don't use GTK or Qt UI widgets other than to paint and get a handle to the main window. They don't even have to use X. See http://wiki.linuxquestions.org/wiki/OpenGL
for a good overview.
Also, check out your options on 3D Engines. OGRE
is my personal favorite. They have a forum where you can get many of your 3D programming questions answered.
Choice of language, in addition to your preferences, depends on the features required by the application, which will determine the maturity of the platform required. I'm attempting a simple OGRE 3D visualization project (map flyovers) using C# on Mono, but I wouldn't build an MMORPG from the ground up using that platform. I would turn to something more like WorldForge
, which is largely C++.
This is the great thing about open source - the majority of wheels have already been invented. If you are flexible and willing to learn, you will find an existing project that almost
meets your requirements, whatever they are. You are then free to modify that project to precisely meet your requirements.