Custom 3-Panel Accordion with Resize Handles

Card 1 body content.

Card 1 & Card 2 can be expanded at the same time (Card 3 then stays collapsed). You can also drag the small 3px bar between panels to resize them.

Card 2 body content.

Card 1 + 2 expanded ⇒ Card 3 collapsed.
Card 2 + 3 expanded ⇒ Card 1 collapsed.
From (1 + 2 open), clicking Card 2 collapses Card 2, Card 1 fills space. Clicking Card 2 again (while 1 is expanded) restores that state.
From (2 + 3 open), clicking Card 2 collapses Card 2, Card 3 fills space. Clicking Card 2 again (while 3 is expanded) restores that state.
Drag handles only resize when both neighbouring panels are expanded. The sizes you set are remembered when you collapse/expand via headers.

Card 3 body content.

Clicking Card 3 header collapses Card 1, keeps Card 2 as-is, and expands Card 3 from the bottom. Drag between Card 2 & 3 when both are open to change their relative heights.