During moments of niavity (sp?), I sometimes think that 3D should be easy and I'm just going about it the wrong way. Surely it's just like 2D, except you have to adjust for depth as well?
Unfortunately, it's nothing like as easy. For a start, with 2D, all your co-ordinates have a common origin, usually the bottom left, and you can tell whereabouts a sprite is in relation to the origin. With 3D, the origin could be anywhere, and usually each model seems to have it's own. In addition, working out where the origin is can be difficult, since it depends on where your camera is and what direction it is looking in.
You might have two 3D models, each facing zero degress (or radians, assuming you haven't got them mixed up again). But one is facing one way and one another. And their direction might change depending on the frame of animation they are in.
And why can't I see anything? Is the camera outside the scene looking outwards again? Or is it looking directly down, which seems to cause problems and make everything disappear? Or are the objects slightly too far away to see? Or maybe I need to add lighting to the scene. And why is one model approximatly 3 million times bigger than the other? Time to scale the models down a bit and then wteak their height so they are standing on the floor and not in it.
And why does my model move whenever I move the camera? Oh, I've forgotten that when I have a unit with a position of Vector vec, and I then do camera.setLocation(vec), it is using the same actual object (vec) for both the unit's location and the camera's location, so when one changes, so does the other. Grrr!