3rd person free is absolutely a thing, but you have to remember that NCL is making their money off of that person on the on-board amenities.
Essentially, what they're doing is allowing them to come onboard for free, in a room where they're already making money on double occupancy, and then hoping to make money off of specialty dining, shore excursions, etc.
Go take a look at a fare comparison of two guests vs three guests. The total price on the base cruise fare for two guests vs three guests won't change. Taxes, obviously, will. Then, you'll start to see additional charges for that third guest, like drink package gratuities, if you choose it.
This third guest then provides them opportunities to gamble in the casino, buy art work, shore excursions, etc. to make their money.
Remember, the rooms are already priced to make money at double occupancy (outside of studios). The only extra costs they really need to cover on that 3rd person is food and any extra staff time they use up (slightly more room steward time?). Overall, fairly cheap. A 3rd person can easily make up that and more with onboard spend, so it's beneficial to let them on for a comped base cruise fare.