Yes, it is. The 3D functionality will be available to all members. Gold Members will be able to create as many 3D models as they like. The only paid service for the 3D functionality is the ability to export STL files from the 3D models you created.
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.
But as far as having the 3D functionality and working with it, it is included with your Gold Plan at no extra cost.