3 monitors is absolutely the sweet spot. On my MBP I use 2 USB display adapters and shut the lid to give me 3 external monitors running, works really well for a number of workflows. To main screen setups I find most useful are
Web Dev:
Web Page - Code - Documentation/Google/command line
and TDD:
Test Code - Code under test - test runner
under Windows they perform really well, no noticeable difference between them and the main screen, on OSX they aren't quite as good in certain applications because their drivers aren't as solid for OpenGL but I barely notice it
