First of all what type of computer do you have? It's not unheard of to tri boot a hackintosh, if you are using a mac than how have you installed windows 7? Using bootcamp or did you manually partition and install? On a mac you need a different boot loader to grub called rEFIt.
If your pc is fairly high spec it might be an idea to install ubuntu inside windows 7, this means you can install windows via bootcamp and than ubuntu inside windows by using the installation cd. This will then use grub when you select you select windows at startup between (windows and ubuntu)
Hope it helps, feel free to ask more