Three 3s in a Row

How it works

1. Read it as two overlapping adjacent pairs rather than as one new pattern.
2. The left pair is two adjacent 3s, so their outer edges are lines: the far-left edge, and the edge between the middle and right cells.
3. The right pair is another two adjacent 3s, giving the edge between the left and middle cells, and the far-right edge.
4. Between them that's every vertical edge crossing the run — four lines from three clues, and the whole run is fenced.
5. Each 3 now has its left and right sides drawn, which is two of the three it needs. So each has exactly one of its top and bottom — the run is fenced, but which way it bulges is still open.
6. A vertical 3-3-3 works identically, with the four horizontal edges.