Hello Pete, and thank you for posting to our Community!
Unfortunately it is not currently possible to sync custom field information from 3dcart, however, I encourage you to repost your request in our Feature Requests forums - these are monitored by our Product Team and are used to prioritize future development work.
Alternatively, if the information in your 3dcart custom fields is product related, such as product options, it is possible to sync your product option information by enabling that in your store settings within ShippingEasy, using the steps in the guide below.
Any Product Options synced to ShippingEasy can be mapped to your Packing Slips using the Shipment Line Item Variable - name_and_product_options, which is outlined further in the link below.
Thank you and happy shipping!