The Deadly Pattern of four cells in exactly two rows, two columns and two boxes can be extended to three rows, three columns and three boxes. This applies to both Unique Rectangle and Avoidable Rectangles.

Consider this set of numbers here. {1,4,8} are solutions in a finished puzzle - the rest of the numbers have been removed. From the point of view of an Avoidable Rectangle the puzzle maker must make at least two of these numbers clues - and two different numbers at that. I've shown in green two example cells that will ensure there is a single solution. If none of these nine cells were clues then the puzzle solver would fine {1,4,8} in all those places and any combination would give a solution. So the puzzle maker is forced to insert at least two clues.

Triple Deadly Pattern

---