I wouldn't usually post here, but since I went to all lengths to get my 3rd panel working, I thought I'd share my experience.
I have an ATI Firepro 4800 card (the pro line not the gaming line), but probably won't make a diff.
Card has 1 x native DVI output, 2 x DisplayPort output.
I initially thought buying DP-DVI adaptors would be easy (isn't it always??).
Only when I found my 3rd panel not working (no display showing although windows 7 could see the "logical" screen there.)
I started to read all I could on Eyefinity. I understood that when going to a 3rd panel, you need an ACTIVE adaptor, otherwise the 3rd panel won't work.
I also called AMD support (who thankfully actually have a phone number which I got from the hardware vendor) who told me that they have a list of "verified adaptors" on their website. Support didn't want to know my problem unless I was using a "verified adaptor".
NOTE: many makes of eyefinity adaptor on the market tell you that they are "eyefinity verified"; AMD's site only lists 3 or 4 makes of adaptor - be ware. AMD's list is growing.
There are two kinds of ACTIVE eyefinity adaptors: non-usb powered ACTIVE, and usb-powered ACTIVE. Theoretically the usb powered ones are only necessary when using a resolution beyond "1080". (I found I needed a USB powered adaptor anyway at a resolution of "1050".
I first tried an AMD Verified (Accell) non-usb ACTIVE adaptor (I couldn't find an Accell adaptor in the UK, found a shop in Holland!!). This adaptor MADE NO DIFF from the non-usb active one I had which wasn't listed as verified by AMD (XFX adaptor).
A note that my panel's symptom was that it hardly was able to show a display -- it was either totally blank, or intermittantly showed its display briefly. Looked to both AMD support & myself like it was a power issue - not enough power getting to the panel.
I next bit the bullet & bought a usb-powered ACTIVE adaptor (@ horrendous cost). This adaptor DID work - to a point. The 3rd panel now displayed _most of the time_. I called AMD support again.
AMD Support informed me that if 3 panels are used, then ALL adaptors NEED TO BE ACTIVE.
At this point I had
1 x native dvi,
1 x PAS adaptor (Bizrate (AMD verified & bundled w the card))
1 x usb-powered ACT adaptor (Accell) installed.
I replaced the PAS adaptor for NON-usb powered ACT adaptor so that my installation now is:
1 x native dvi
1 x non-usb powered ACTIVE adaptor (Accell)
1 usb-powered ACTIVE adaptor (Accell).
Setup finally Bl**dy works. I also have AMD's Catalyst Control Panel setup for GPU scaling - Maintain aspect ratio. (Catalyst: My Digital flat-panels -> Properties -> Image Scaling.) I'm using panels of 2 different native resolutions. Without 2 x ACT adaptors, this aspect ratio setting didn't work.
Hope this helps someone, was a nightmare experience for me!