So, perhaps this is the new workflow but I noticed that when you create a new row and choose a column layout then change that layout to include more columns you need to manually add another column.
Example:
- Click Add New Row
- New Preview Column Layout Selector Appears
- Select a 2 Column Layout
- Change your mind and click 3 Column Layout in the inspector
- Still only shows 2 editable columns, a third must then be added via the inspector.
The way it used to work was change layout and the columns just appeared. Feature? Bug? Just curious.
Video in note below.