Short answer

The "3 Days" on a replica Luminor Due is a copy of the power reserve marking Panerai prints at 6 o'clock on the Luminor Due 3 Days Automatic. The genuine watch is Panerai's slim, dress-leaning Luminor: a 42mm or 45mm case (38mm on later references), roughly 10.5mm thick in the 42mm version, sandwich dial, small seconds at 9, date at 3, and a 72-hour automatic inside. The dial text is the easiest part to reproduce. The case profile, the crown guard, and the movement are the parts that separate a good copy from a bad one.

What "3 Days" really refers to

It is a power reserve figure, not a model name. Panerai introduced the Luminor Due in 2016 with three-day automatics and later moved the line to the P.900 family, both rated at 72 hours. Replicas rarely run a true three-day movement. Most use a Miyota 9015 or a generic Asian automatic with a 40-hour reserve or less, then print the text on the dial anyway. On most Luminor Due references the caseback is solid, so the movement is hidden. That makes the claim cheap to fake and hard to verify without opening the watch.

Case size and thickness

This is the number I check first. The whole point of the Due line is the thin case, and thin is expensive to copy. A genuine 42mm Luminor Due sits near 10.5mm. Many replicas land between 12mm and 13.5mm because the movement and spacer stack take up more room. Measure with calipers across the center of the caseback rather than at the bezel edge, since a domed crystal and a beveled bezel can hide height. A half millimeter is normal tolerance. Two millimeters is a different watch on the wrist, and it changes how the watch sits under a cuff.

Crown guard and lever

The Luminor crown bridge is the hardest piece to get right. On a good example the lever closes with a firm snap, sits flush against the guard with no visible gap, and the brushing runs in one consistent direction. On cheap copies the lever feels soft, sits proud of the guard, or rattles. Check the engraving on the guard too. Letters should be crisp and evenly spaced, not shallow or doubled from a worn stamp.

Dial, hands, and crystal

The Due uses a sandwich dial: two plates with cutouts filled from behind. Replicas often fake this with printed lume or shallow pockets, so look at the cutouts from an angle. The date font and window placement are common tells, along with the spacing and weight of the "3 DAYS" text. Hands should be slim batons with clean edges. Ask whether the crystal is sapphire with an anti-reflective coating, or mineral glass. Mineral scratches and looks slightly hazy at an angle.

Water resistance

Genuine Luminor Due 3 Days references are rated to 30 meters, about 3 bar. That covers rain and handwashing, nothing more. Replicas carry printed ratings that nobody has tested. I treat any replica as splash resistant at best and keep it away from swimming, showers, and salt water.

Checklist before you commit

• Ask for a caliper measurement of case thickness, photographed on the watch.
• Request a short video of the lever opening and closing.
• Look at a straight-on dial shot for date font, subdial position, and text spacing.
• Confirm the crystal material and coating.
• Check strap hardware: buckle finish, tang shape, and stitching.
• If the caseback is solid, ask what movement is inside and who assembled it.

One legal note

Counterfeit watches are illegal to import into the United States and can be seized in transit, and reselling one is a separate problem. That is worth knowing before spending money, because a replica is a wear-it-yourself object with no resale market and no service path from the brand.