As FedEx do not support address line 3, you'll receive an error on any shipment that is sent to them with an address line 3;
Something unexpected happened and unfortuntely we don't know why. If this continues to happen, please contact Shiptheory support.
Any shipments that have already failed due to this can be easily resolved by (re)moving the 3rd address line, to prevent this from happening to any future shipments you'll want to either edit your checkout and/or how your channel manages the address information.
If you have any questions about this feel free to contact support.