Note: By default, the order you write your HTML is the order elements appear on the screen. Nonetheless, using the order property, you can display side-by-side flex-items in any order you’d like. See MDN

container1
Sidebar No. 1 order 1
Main content area order 2
Sidebar No. 2 order 3
container2
Sidebar No. 1 order 1
Main content area order 2
Sidebar No. 2 order 3
container3
Sidebar No. 1 order 1
Main content area order 2
Sidebar No. 2 order 3
container4
Sidebar No. 1 order 1
Main content area order 0
Sidebar No. 2 order 0
container5
Sidebar No. 1 order -1
Main content area order 0
Sidebar No. 2 order 0