The short answer: I’d go with a folding aluminum-and-sling chair or a compact roll-top aluminum table. There’s no single “lightest” piece that works for everyone, but those two types are about as light as practical urban outdoor furniture gets. A good sling chair can weigh around 4–9 lbs, and a small aluminum table often lands around 5–12 lbs; a folding side table or camping-style stool can be 2–6 lbs.
For three flights of stairs, weight matters, but so do folded dimensions and how you carry it. Look for a chair that folds flat or into a slim bundle, has a built-in carry handle or shoulder strap, and has no sharp metal ends that will catch on railings. Check the folded length against your stairwell and doorways before buying—sometimes a 10-lb chair that folds long is more annoying than a 12-lb chair that folds compactly.
Materials make the biggest difference. Powder-coated aluminum, weather-resistant fabric, and high-density plastic are your friends. I’d avoid cast iron, solid teak, glass tops, ceramic, and anything that needs two hands and a wide turn on the landing. If you want a table, choose an aluminum roll-top or a small folding bistro table; if you mainly need a seat, choose a sling chair or a low-profile folding stool. A soft-sided outdoor pouf is light but not great as a table.
If you’re carrying everything in one trip, keep the total load under roughly 15–25 lbs and use a shoulder strap or a small folding cart if the stairwell is tight. My pick for the lightest realistic setup: one folding aluminum sling chair plus a compact folding side table. It’s light enough to haul up three flights without dreading it, and it still feels like real outdoor furniture rather than a temporary campsite afterthought.