When you first add a Column Group to the page it will consist of 3 empty columns:

You can then drag Content Cards into each individual column:

Individual Column Menu

The blue Column menu appears when you click on a column within the Column Group:

These are the available options in the Container Box menu:

Background Color
Give the column a background color to help it stand out on the page.

Padding
If you want to increase or decrease the amount of space between the edge of a column and the contents within it you can use the padding controls.

Large padding:

No padding:

Borders
To add borders around a column, select an option from the Border Style list and set the Border Width on each side to at least 1.

Border Color
You can change the color of the border with this option.

Reorder
You can move a column left or right within a column group by using the arrows at the bottom of the menu: