A three column layout with main column in the center and two stack columns surrounding it. There will be always a pane in the center column and unoccupied spaces on the sides are reserved. It's best suited for ultrawide montiors, where a single stretched window might be annoying.

You can use this module by adding following in your xmonad.hs:

import XMonad.Layout.CenterMainFluid

Then edit your layoutHook by adding the CenterMainFluid layout:

myLayoutHook = CenterMainFluid 1 (3/100) (70/100) ||| ...
main = xmonad def { layoutHook = myLayout }

The first argument specifies how many windows initially appear in the center column. The second argument specifies the amount to resize while resizing and the third argument specifies the initial size of the center column.

For more detailed instructions on editing the layoutHook see the tutorial and XMonad.Doc.Extending.