Columns sit in rows, which can also be in rows. To put a full width column above a row with multiple columns, you need to select the full width row that holds those columns, then insert the full width column.
Here's a page with three columns:
Select one, and then select the levels menu on the left of the toolbar. The levels menu says the name of the module you selected at the top.
Select the first row that contains all the modules above which you want to play the full width row. In this case, it's the top row, but in other cases, there might be more rows. When you select the right row, it puts a box around the modules and is named Row
Click the up arrow in the tool bar to add a module above this row...
...then choose 100% (full width) layout module.