I've heard conflicting things. Some say yes, others say no.
I can get 2 to run, but the third seems to be completely invisible to Windows XP.
I'm running 2 5850's and a 5870. I can only get Windows to see 2 at a time. I have my monitor on 1 and dummy plugs in the others.
It works for me... Im running 4x Sapphire 5850 xtremes. Only thing I can think of is that you havent extended the displays on the other card, so it's still technically disabled. Go in to the display properties and add a display to the side, that's what it took for me to get it to work.