Your PCB has a defined outline. Assuming you have a rectangular PCB you can simply read its dimensions in the "Set Board Outline" menu and create a CAD model of it. For non rectangular boards you have to use the ruler tool and manually reconstruct you PCB outline in your CAD. For the layout to be on the model you can take a screenshot of it in the 2D view and set it as your models texture. If you need the components too, you currently can't export it. But due to the open nature of most component models you can add them manually if you need to.

I think you can export to altium (I don't know why they don't export to kicad that is open source), use web converter from altium to kicad, open pcb in kicad edit component and export stl. (you should first update the libraries with easyeda ), I think that if it is a difficult pcb it is better to do it directly in kicad

At present, you can export DXF on PCB, 3D model doesn't support yet. it will be support next year. we doing a more important job now, a new PCB engine, which is more faster, even you have more than 50,000 pads on the PCB