Ok, I''m working on an RTS engine, and I''m getting near the part where I have to think about the map and moving around in it. I can understand how it works if its movement along a single plane, but what if you want totally 3D movement, like homeworld style, how do you go about that? Ideally I''d like to keep movement 2d but leave the option open (this is an engine im programming) for full 3d movement.

Let each object move in the XY-plane and move them in the Z-axis depending on the height of the terrain at that point. Later on, you could try to tilt each object based on slopes in the terrain
But first, make sure you get 2D-movement going.