3 Skyscrapers
3 Skyscrapers chains three parallel strong links on the same candidate. Each link confines the digit to two cells in a row or column, and consecutive links share a bridge cell on a perpendicular line. The chain forces at least one of the two unlinked endpoints to hold the digit, eliminating it from any cell that sees both.
What is 3 Skyscrapers?
3 Skyscrapers is a single-digit pattern built from three strong links running in parallel — three rows, or three columns, where a particular candidate appears in exactly two cells. The six cells form three pairs, and one cell from each neighbouring pair lies on the same perpendicular line as its partner. Those shared-line cells are the bridges: because each pair of bridge cells sees each other, they can't both hold the digit. Since each strong link must place the digit in one of its two cells, and the bridges can't supply both ends of the chain, at least one of the two outer endpoints — the roof — has to contain the digit. Any cell outside the pattern that shares a house with both roof cells can't also hold the digit, so it gets eliminated.
How to spot it
3 Skyscrapers is a natural extension of the two-link Skyscraper, so you find it the same way — by scanning one digit at a time across parallel houses. Pick a candidate digit and look for rows (or columns) where it's restricted to exactly two cells. When you find three such rows, check whether one cell from each neighbouring pair shares a column with the next. If they do, you've got a 3 Skyscrapers chain: the shared-column cells are the bridges, and the two cells at the ends of the chain are the roof. The elimination lands on any cell that sees both roof cells — same row, same column, or same block as each. These patterns are rarer than two-link Skyscrapers, but they turn up in harder puzzles once the easier strong-links techniques have been exhausted, and they're worth checking whenever you see three parallel strong links on the same digit.
The logic
Each strong link is a house where the digit has exactly two candidate cells, so one of them must be the digit. In a 3 Skyscrapers chain, one candidate from each neighbouring pair sits on a shared perpendicular line — the bridge. Because the two bridge cells see each other, at most one of them can be the digit. If the first bridge cell is the digit, the second link's digit falls on its roof cell, and the chain stops there. If the first bridge cell is not the digit, the first link's digit falls on its other cell, which is also a bridge to the next link — and the same reasoning carries forward. Tracing the chain link by link, every case forces at least one of the two outer roof cells to contain the digit, so any cell that sees both roof cells can't also hold it. The same reasoning applies with rows and columns swapped.