MacBook Pro Retina running Bootcamp with 2 external 27" monitors
Solution 1:
If your MacBook Retina is running in 150 DPI then your external screens will run in 150 DPI as well. This is a no go.
You can run your MacBook in eg. 1920x1200 (or any other resolution). But because this is not native resolution it is grainy. Even running with perfect pixel doubling like 1440x900 is blurry (http://apple.stackexchange.com/questions/59283/why-is-a-retina-macbook-running-1440x900-grainy-under-boot-camp). Like going from none-retina to retina (just the other way).
I have been running with the retina under Boot Camp for many weeks now, and I've given up. Yesterday I ordered a MacBook Air and I'm selling my retina on eBay (Denmark). I'm pretty tech savvy... And everybody who says retina and Windows are great lies!
You can try this on your own machine before buing a retina. Just change the DPI on your current laptop and attach an external screen and try changing the resolution to a none native resolution.
If Windows are your main OS, don't buy retina! Period.
Solution 2:
I have a 2 x 24" setup at work and Windows 7 via Bootcamp. You can't have all three displays (2 external and 1 MBP) usable at the same time since Windows won't allow setting a different DPI on a per-display basis.
What you can do, however, is set resolution and DPI to work for external displays and use MBP display as an auxiliary space, maybe to move windows out of the way or what have you. Everything will look tiny on it, but you could find some use for it.