do you have the video card with 3 outputs? 1 - DVI and 2 - Mini Displayport? I think it's called Radeon 5770 or something like that. it's the default one in newer macs.. 2010+
I fell into this trap in the past, so hopefully this helps save you time and money - don't think that you can just run 3 displays off of it. learn what active and passive display adapters are. basically, there's a combination of each to get the 3 displays to work. Google around and you'll find others who find working combinations.
I use 1 DVI and 2 Mini Display to VGA adaptors. DVI goes to my main sequencing monitor for high resolution. My second monitor is VGA and it is used to show VEP or plugins or whatever, so resolution isn't key. And for my 3rd monitor, it's for video - VGA, and I use a lower resolution, even though it's a 60" TV, but the videos still look great!
Check compatibility with 2008 models.. i think my MP is 2009. I think there's something you have to do when installing it.. like plug a power cable somewhere different on the mother board.. check it out!