The first step in connecting up 3dcard and Shiptheory is to access your "Manage Channels" page in your main Shiptheory dropdown:
Within this page, you'll want to find 3dcart and click the plus sign to add it to your active Shiptheory channels:
You'll then be faced with this page, where'll you'll want to click "connect":
This will then take you through to 3dcart itself for the authentication process where you'll need to input your details and confirm Shiptheory is able to connect to your store.
Once you have authenticated everything, you will be redirected back to Shiptheory, where you should see a green confirmation that your connection is successful:
Once you're connected, you'll then have access to change your 3dCart settings in Shiptheory. Although these settings will be addressed in these guides; Bringing Orders Through, Commodity Data and Dispatch Dates
If you have any issues or questions regarding this, or would like any help getting set up, please feel free to contact support.