A layout similar to tall but with three columns. With 2560x1600 pixels this layout can be used for a huge main window and up to six reasonable sized slave windows.

You can use this module with the following in your xmonad.hs:

import XMonad.Layout.ThreeColumns

Then edit your layoutHook by adding the ThreeCol layout:

myLayout = ThreeCol 1 (3/100) (1/2) ||| ThreeColMid 1 (3/100) (1/2) ||| etc..
main = xmonad def { layoutHook = myLayout }

The first argument specifies how many windows initially appear in the main window. The second argument argument specifies the amount to resize while resizing and the third argument specifies the initial size of the columns. A positive size designates the fraction of the screen that the main window should occupy, but if the size is negative the absolute value designates the fraction a stack column should occupy. If both stack columns are visible, they always occupy the same amount of space.

The ThreeColMid variant places the main window between the stack columns.

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