Are you just a noob to MonoGame, a noob to programming at all, etc?
It's largely the same as anything with 2D, just now you have to deal with Quaternions (called rotators in Geometric/Clifford Algebras) instead of wanton vector rotation, etc. Stuff that was previously mostly hidden or fudged is now in your face - that's really the only difference between 2d and 3d.
There's so much grunt-work involved in a 3d anything demo for a relatively raw library like MonoGame that I'd think your best bet would be to grab the BEPUPhysicsDemo for MonoGame and mess around with that.
Edit: if you answered twice to the "noob" questions above maybe check out Urho3D? I wrote the DetourCrowd and initial PBR for it, it's not as raw but still pretty metal.