Everything that is essential for 3D is already in Allegro 5.1 - projections, 3d transformations, shaders (GLSL or HLSL), etc. You don't need any external plugins or addons. All functionality wraps OpenGL and DirectX so you don't have to interface with either of them directly (but you can if you want to).
There are a few really good example programs in Allegro for 3D, namely ex_projection, ex_projection2, ex_camera, ex_shader, are a few.
That being said, right now there aren't a lot of tools in Allegro 5 to make 3D programming easy. Everything takes a bit of work and patience. Since Allegro is so low-level, it doesn't have functions to load 3D model files (.obj for example), or the concept of 3D models at all. But you can create whatever model format you wish, render as many triangles as you need in whatever way that you want.
You might also want to check out AllegroFlare, which I'm still working on. In there is a framework with a 3D pipeline and a Model class that loads .obj files.