Ushuaia is the end of the road, and not much else about it is an ending. It was founded in 1884 on the Beagle Channel, became a mission base, a prison town and a naval base, and today it is a tourist port of around 90,000 people with a mountain wall behind it. Chile's Puerto Williams across the water also claims the title of southernmost city, but it has fewer than 3,000 residents. Ushuaia is 3,260 km from Buenos Aires and one of the main gateways to Antarctica.
The weather is moodier than the postcards. Summer days are around 13°C with sun, cloud and rain showers in the same hour. Three days is the right length because almost everything good sits within a 20-minute drive of the center, but each big thing (the park, a boat, the penguins) takes half a day or more, and weather can move one of them. Two days would mean dropping something you came for.
The plan: day one is Laguna Esmeralda in the morning and the Beagle Channel cruise in the afternoon, day two is Tierra del Fuego National Park and the old prison, day three is Martial Glacier and the penguin walk, the one trip that needs booking ahead. Start on a Monday so day three isn't a Tuesday (more on that at the penguins). The season is November to March; in winter the same plan becomes a ski trip, see Cerro Castor under more time. Ushuaia works as a first or last stop in Argentina, and it pairs well with El Calafate, the other half of the Patagonia trip.