# Inertia Tensors in 3D objects for robot dynamics

I'm trying to obtain the dynamics of a 6DOF robot. Firstly, I calculated the combined centres of mass (between each link and the respective actuator) in order to calculate the gravity term, since it only depends on the combined centre of mass the the angles of the joints (Yes, I know there's another way which basically takes into account the centre of mass of the links and the actuators, but I just didn't follow that path). My calculations are correct since the robot is effectively compensating the gravity, but now I want to calculate the remaining terms (mass and coriolis).

Since I have the combined centre of mass between each "system" (link + actuator), and the datasheet of this robot only gives me the Inertia tensors at the center of mass of each part, I need to know the equivalent inertia tensor in the "new" centre of mass (the combined one).

Now, I've done my research and found little information on the web about this. I did found out that I could probably follow the parallel axis theorem, but I've seen people saying that it is based on the premise that the object is planar (which means it would only be applied to 2D objects). My question is: can I apply this theorem in 3D? If so, please explain to me what exactly do I have to do, and if not, what other options I have to follow.

Let me know if you understood what I'm after and thanks in advance.