The Tridente is Rome showing off its geometry: three dead-straight streets fanning south from one square, laid out so a pilgrim arriving from the north would feel the city open up in front of them. Walk it the same direction and it still works.
Start in Piazza del Popolo, under the obelisk Augustus brought back from Egypt. Duck into Santa Maria del Popolo first — two Caravaggios in a side chapel, free, and almost nobody looking. Then pick your prong. The middle one, Via del Corso, is the shopping spine and the obvious choice; I'd take the eastern one, Via del Babuino, for the antique shops and the calm.
Either way you land at the bottom of the Spanish Steps, with Bernini's sunken-boat fountain in front and Trinità dei Monti at the top. The climb is the famous part, but the quiet stop is the Keats-Shelley House on the right — the room where Keats died, and the window looking straight down the Steps he never recovered enough to climb. Spagna on Metro A is below you.