Main panel with columns below

The three panels below the main panel render in a column on small screens. Swap in md or lg for sm to switch to a column layout on larger screens.

Main panel

Column 1

Column 2

Column 3

Further reading

Use display() with type = "flex" and flex() to use flex box layout. See https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout/Basic_Concepts_of_Flexbox for more information on flex box layout.