As far as having the 3D functionality and working with it, it is included with your Gold Plan at no extra cost. 

The only cost for the 3D functionality is the ability to export STL files for cases. 

Once you pay the fee to export the STL files for a patient, you will have unlimited STL exports for that patient only. 

If you would like to create and export STL files for other patients, you will need to purchase 3D cases for them, as well.