It depends where the content is, but you have two options
1) Using the primary / secondary Widget area settings, you can use the Custom Widget width boxes to make the rows of three columns, like for two rows of three.
33.3, 33.3, 33.3; 33.3, 33.3, 33.3;
and combine that with the options “Add side margins” and “Equal height widget rows”
Then we can use custom CSS to do rounded borders
Complete guide of the widget area settings
2) Or use the flex column widget following the tutorial below