I wonder about its viability compared to the other two.
Well, I've heard several stories recently about fairly major studios switching over to Lumberyard. I would agree that Unity and Unreal Engine 4 have a pretty firm grip on the indie scene. I haven't heard much about indies switching to Lumberyard. It is based off of CryEngine, which is feature rich but perhaps not nearly as user-friendly.
I would say that Lumberyard seems to be gaining traction with mid-tier developers. And Amazon themselves are clearly not intending to bail on their new game initiative. They just announced the formation of a new Amazon game studio in the San Diego area that will be using Lumberyard for their projects.
As to the specifics of Lumberyard, I can't really say. I'm still using Unity.