Are there any applications out there for 3D modeling that would output the finished product to an array of points (X,Y,Z) that you could then import to a WPF 3D Model?
ie:
<MeshGeometry3D Positions="0,0,0 10,0,0 0,10,0 10,10,0 0,0,10 10,0,10 0,10,10 10,10,10"
TriangleIndices="0,2,1 1,2,3 0,4,2 2,4,6 0,1,4 1,5,4 1,7,5 1,3,7 4,5,6 7,6,5 2,6,3 3,6,7" />
A simple google search for "WPF exporter", an I found:
Viewer3ds - 3ds to xaml converter
Viewer3ds is a WPF application that can read 3ds files and convert them into xaml. It is using the Ab3d.Reader3ds library that imports 3ds files at runtime.
This might be what you need because almost all 3D authoring application are able to export to .3ds format.
Edit:
You could also try Blender and XAML Exporter for Blender to create your 3D assets.
Viewer3ds is a converter rather than a modeling tool. If you are looking for free tools, Blender is a very powerful 3D modeling application (albeit with a high learning curve for the UI and workflow) which can export .xaml files with the help of this Blender plugin.
If free is less of a concern to you, then you can use other applications such as Maya or 3ds Max, also with plugins : http://max2xaml.codeplex.com/
All tools mentioned above have the ability to define normals, smoothing groups, materials, etc., everything that WPF supports (and much more, of course).
you can use google sketchup, and the exporter I wrote.
http://itaibh.blogspot.com/2009/09/google-sketchup-xaml-exporter.html