A three-day solo trip is a different proposition from a long one. It is short enough that the awkwardness barely sets in, and short enough that the destination has to be small.
Small beats famous
On three days, the constraint is internal transport.
A compact city whose centre crosses on foot in twenty minutes removes it entirely. A large capital spends the time on the metro.
The other constraint is the airport transfer, which on a short trip matters more than the flight length. Twelve minutes versus seventy-five is two and a half hours of a three-day trip.
The cities that suit it
Porto — the Douro, port lodges across the river, tiled churches, and the best value in western Europe. Hostels with communal dinners.
Ghent — the less-visited counterpart to Bruges, with a working population and a canal centre.
Ljubljana — small, riverside, walkable end to end, with a castle above it.
Kraków — an intact medieval square, Kazimierz, and very low prices.
Bologna — porticoes, a student population, and food as the organising activity.
Seville — walkable, warm, and cheap outside the festival weeks.
Tallinn — a walled medieval centre you can cross in twenty minutes.
Valencia — a river-bed park, the old town and the beach, at a third less than Barcelona.
What to book
Accommodation — a hostel with common areas if you want company, wherever you take the private room.
One activity for the first full day — a free walking tour is ideal, since it orients you and puts you among people at once.
Dinner for the arrival evening.
That last one is the single most useful preparation. Deciding where to eat at nine at night in an unfamiliar street is how first evenings get wasted, and it happens on almost every unprepared solo trip.
The shape of three days
Arrival evening: drop bags, walk the neighbourhood, eat somewhere you chose in advance.
Day one: the walking tour in the morning, then whatever it made you want to see. Lunch as the main meal.
Day two: one thing you booked, then a district on foot with no plan.
Departure morning: store bags after check-out and use the morning rather than sitting on it.
What three days does not do
It does not get you fully past the initial strangeness, which usually takes three or four days.
A first solo trip is usually better at about a week, and a three-day version can end while it still feels odd.
That is worth knowing rather than treating as evidence that solo travel is not for you.
Practical
Hand luggage only — saves 30–45 minutes at each end.
Check the airport transfer time before booking a cheap flight into a distant secondary airport.
Stay central, even at a premium. On three days, the walk saved is worth more than the money.