Hi it came with no box and serial number is gone tomorrow I will post some pics 
Amazon has a dispute resolution department through customer support. If you are intent on returning the device, I recommend that you: (1) tell them that D-Link stated that no DSM-750s were manufactured with 1 antennae; (2) forward excerpts of the DSM-750 manual with images and text referencing 3 antennae; and (3) send images of your device with one antennae, pointing out any other differences in the outer casing.
You can concurrently challenge the charge through your credit card company using the same evidence.