In ICS 4.0.3 using ADWLauncher EX, how can I make only three home screens instead of five?
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.