http://www.web3d.org/x3d/opensource/Some/most of these can handle openGL.
I think most of them can handle dxf,obj, and 3ds
If someone is actually interested in getting the ball rolling with a viewer, I don't mind sifting through the code to find that part for any of these. One is a group of libraries which includes a viewer. Many of these also have active X options. I'm not sure if that would help as far as structuring the code.
• Xj3D is a Java-based toolkit and X3D browser for creating X3D-compliant products. Xj3D is often used to prototype new extensions and features for X3D. It is highly componetized and can be used as the basis to develop lightweight X3D applications.
• FreeWRL is a VRML/X3D browser for Mac OS X and Linux with support for Javascript interfacing, the External Authoring Interface (EAI), and the X3D Scene Authoring Interface (SAI). ***there is a windows version
• OpenVRML includes a cross-platform VRML/X3D runtime library written in C++ and available for use under the LGPL as well as a Mozilla browser plug-in for platforms using the X Window system.
X3DToolkit is a portable LGPL free C++ toolkit for loading, displaying and processing the X3D scenegraph and models.
Included example applications include an X3D viewer and mesh editor.libx3d is a set of libraries and tools to work with XML X3D data/files and rendering using OpenGL.