I almost agree with you, but - isn't 'Tx TAG' the same as 'Trunking'?
Tx commonly stands for 'transmit' or 'send'.
I'm wondering if 'Tx TAG' checked means 'pass all traffic tagged, and tag untagged traffic with VID'.
The PORT MAPPING SETTINGS section of the GUI obviously only allows to specify exactly one VID per PortX. VLAN-Trunking means to define at least two or more VIDs per PortX where any Ethernet frame that enters or leaves the port has to be tagged with a VID with the exception of exactly one VLAN (the 'native' VLAN), whose frames may be sent and received untagged (and which are internally tagged with the native VID). As a consequence your DSL-2770L does not support VLAN-Trunking (apart from the special case that you may consider a port with a single configured VID, that sends and receives tagged frames, as a VLAN-Trunk that consists of a single VLAN).
Tx Tag if checked means: Egress frames are tagged with the VID configured for that port and ingress frames are only accepted if they are also tagged with the VID configured for that port. You can consider this port as a VLAN-Trunk that consists of a single VLAN.
Tx Tag if unchecked means: The configured VID is only used internally: Ingress frames are only accepted if they are untagged (and after ingress they are only internally tagged with the VID configured for the ingress port) and egress frames are sent untagged (the internally used VID gets removed on egress). The corresponding port is commonly named an 'access port'.
EDIT: Maybe the DSL-2770L supports a special feature that any port configured for a VID > 1 with Tx Tag checked can be considered as a VLAN-Trunk consisting of two VLANs, where the default VLAN 1 is implicitely and unchangeably taken as the second and native VLAN of that VLAN-Trunk. If so your scenario might work if you configure port 3 for use of VLAN 1003 with Tx Tag checked.