Triangulation registration space

Hello all, I have a few questions regarding triangulation. For starters, I have set up 3 cameras (all perpendicular to one another). I want to triangulate the camera outputs (I understand that triangulation cannot use all 3 camera at this time, but eventually that is something that will happen), but have some problems interpreting the output.

How is the 3D space registered? What is the “x”, “y”, and “z” assigned to? Are different “scans” co-registered to a common space?