When resized the three pane layout only displays one pane and does not allow for a user to access the other two panes – the responsive layout doesn't work properly.

See attached file of areas with this issue. 

Violates: 1.4.10 Reflow (Level AA 2.1 only) - https://www.w3.org/TR/WCAG21/#reflow:

Content can be presented without loss of information or functionality, and without requiring scrolling in two dimensions for:

• Vertical scrolling content at a width equivalent to 320 CSS pixels;

• Horizontal scrolling content at a height equivalent to 256 CSS pixels.