I was unable to find information on if the 6870 is even capable of using 3 screens simultaneously.
Technically, every Radeon HD 5000 series and up GPU is capable of eyefinity. But that doesn't mean that every card with one of those GPUs will support it. In Sasquatchreap's case, it does - I'm not aware of any 6870 card that doesn't have a DP port and a couple of other connectors. But if you look at the Radeon 5450, you'll see that most of them don't have a DP connector and therefore can't do more than two monitors. There are a few models of the 5450 that do have a DP port and a couple of others and can do eyefinity (though it wouldn't be worth a hoot for gaming).
Have you tried a complete removal of your graphics drivers and a reinstall?