Yes, three days is the right length for Barcelona, and it is what we recommend for a first visit. It gives the Gaudi trail a full day (Sagrada Familia, the Passeig de Gracia houses, Park Guell), the old town its own day (the cathedral, El Born, the Picasso Museum, La Boqueria, La Rambla), and a third day that mixes the beach with the modernist concert hall and Ciutadella park at an easier pace. You eat well across the Eixample, El Born, and Barceloneta, fit both a proper beach afternoon and a sunset, and still sit in a plaza rather than sprint past it. What still gets cut: Montjuic in full and any day trip, which is exactly what a fourth day adds. This suits first-timers, couples, and anyone routing on to Seville or Granada next. Stay in the Eixample: it walks to the Passeig de Gracia Gaudi houses, sits close to Sagrada Familia, and its metro grid makes the old town and beach quick to reach. Pre-book Sagrada Familia, Park Guell, and the Passeig de Gracia houses weeks before you fly, and keep your bags zipped in the old town and on the metro.