You can aggregate the geometries of each layer and than combine them using `collect_geometries()`. The final expression could look like this - replace `'poly1'`, `'poly2'` etc. with the names of your polygon layers:

```
collect_geometries( 
aggregate( 'poly1','collect',$geometry),
aggregate( 'poly2','collect',$geometry),
aggregate( 'poly3','collect',$geometry)
)
```
*Screenshot 1: original polygons (3 layers):*
[![enter image description here][1]][1]

*Screenshot 2: the collected polygons with a common line-pattern:*
[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/qgsLR.png
  [2]: https://i.sstatic.net/cYgqB.png