I just upgraded my android phone to ICS. When I had Gingerbread (2.3), I remember the option to change the number of home screens. I also had ADW launcher in 2.3 before. Now I can't find the option in ICS to change the number of home screens.
From the Homescreen, go to Menu, then Edit. This brings you to an overview of configured home screens. Long-press the one you want to remove, and drag it to the trashbin which appears. Continue until only 3 screens are left.
Similarly, if you want to add more screens, there's one "screen" with a plus-sign ("+") in the same place. Tap that one, and you'll get an additional screen.