openpyxl: left right border for merged cell

Question

The documentation says need to change only the top-left cell of merge cells.

ws.merge_cells(start_row=1, start_column=1, end_row=1, end_column=3)
ws.cell(1,1).border=Border(right=Side(border_style='thick', color='000000'), left=Side(border_style='thick', color='000000'))
wb.save(filePath)

This does not change the right side of the merged cell border.

Any idea/help?