remember that the competition for the 3870 is the 9600GT not the 8800GT, the 8800's and faster that a 3870, so in a rig with 2x3870 vs 2x8800gt the 8800 GT's would win.
EDIT: oooooooh you mean a 3870X2 + 3870 (3 videocards) vs 2 x8800GT's (2 videocards).
let me do some research ill be back....
why would you ask? you have an SLI motherboard. But the the 3870 X3 will only be faster in games that it scales will in. overall I think the 8800GT SLI will be faster, and use a lot less power.