To authorize 3DCart to send emails on your behalf you will have to include it in your SPF record. The SPF record mechanism used by 3DCart is shown below.
In order to use DKIM with 3DCart, you need you must use a domain for your 3DCart store. It will not work for stores only using the *.3dcartstores.com URL.
DKIM can be enabled in your 3dcart Online Store Manager / Modules.
Create a free OnDMARC account to test your configuration.