i just set up 2 GTX 280's and can confirm that a 3-way SLI bridge will work!!
be sure to install latest drivers and REBOOT!! my 2 cards mem did not register
until i rebooted.. and.......
yes!! i love it!! but i was not prepared for the heat, you will certainly need to bump
the fans up
hope this helps,
i7 940 (@3.4 Ghz) vista 64xx
P6T 6 GB dominator 1600
2x GTX 280