1. Open FBL5N (or any report you use). Fill company code, date range, open items.
2. Use the yellow arrow (multiple selection) to enter 3 customers and exclude one.
3. Save as variant 'MY_OPEN_ITEMS' with a description (Goto > Variants > Save as variant).
4. In the variant, set a dynamic date (Selection variable 'D' for current date).
5. Run it in background (Program > Execute in background) and find the output in SM37 / SP02.