3 content boxes

You don’t say what you want to go in the boxes or on what page types.

In general, though, three left or right floats, each one a third of the width of the parent block will do it, although you may have to make some adjustments for paddings, margins and borders.

On the business of child themes, it takes less than five minutes to create a child theme that behaves exactly like its parent:

• Create a folder
• Create a file (style.css)
• Copy and paste the required text into the style sheet
• Modify the two or three lines of text needed for your particular situation
• Import the parent style sheet
• Activate the child theme

Making changes to the parent theme, especially a default theme, is more than a bit risky in my opinion; and it also means you have much more work to do if you later decide to use a child theme after all.

Just my 2c (dwy geiniog Cymraeg)

HTH

PAE