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.

include:spf.protection.3dcart.com

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.
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Create a free OnDMARC account to test your configuration.