Three monitor setup for gaming using two 3870's in xfire.

You can not use more than 2 displays with crossfire on those cards. With the 5000 series you will soon be able to do that (up to 6, maybe more) when they update the drivers for eyefinity for CF.

However, with that setup you can get something like this and game across up to three displays:

http://www.matrox.com/graphics/en/products/gxm/

There is a software fix too, SoftTH, than works similar as Triplehead2go (although not as well and more complicated)

The problem you have here is 2 fold. You're running the 3870s, which are currently 2 generations behind, while gaming on 3 monitors which the size makes me assume you would like to game at 1920*1200 apiece ??

The higher the resolution, the harder it will be to push any good games with those cards.

I would recommend trying SoftTH for free, if it works for you, great, if not, it doesn't cost anything.

IMHO though, you are probably going to want to stick with the 28" monitor for gaming until you have a new 5XXX series card that supports Eyefinity.

In summary, NO, you can't use 3 monitors with CrossfireX enabled, YES you can use games without triplehead2go but not easily.

yes.

edit: make sure one of your montiors has DisplayPort, if none of them have it then you will need to buy an additional Displayport-to-DVI active adapter which will cost you another ~$100.