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

 import XMonad.Layout.ThreeColumns

Then edit your layoutHook by adding the ThreeCol layout:

 myLayouts = ThreeCol 1 (3/100) (1/2) ||| etc..
 main = xmonad defaultConfig { layoutHook = myLayouts }

For more detailed instructions on editing the layoutHook see:

XMonad.Doc.Extending