Well, I've got some news: managed to do some tests with a displayport cable and a dvi/hdmi cable. I used two dell 32" monitors (they have vga, dvi and displayport connectors) and here are the results:
- dvi + hdmi: worked perfectly, full 1080p on both monitors with GraphicsEnabler=Yes. Didn't notice any issue at all.
- dvi + displayport: it works with GraphicsEnabler=Yes, BUT only if I connected the displayport cable after logging in. If the cable is connected before logging in the monitor connected on displayport displayed a warning about having no connection and went into sleep. Unplugging it, logging in and plugging it back made it work, but it is quite annoying. I tried setting PciRootUID=1 and 0, and AtiConfig=Duckweed but nothing helped. Perhaps my friend got a bad cable, but I will only know when my adapter arrives.
We didn't have 3 monitors to test but when we connected the 3 cables (dvi, hdmi and displayport) it behaved as if there were 3 monitors connected (I got to the login screen and the two monitors were displaying a gray background, meaning the login window was supposed to be shown on the third monitor). So I think that it is very likely that it will work with 3 monitors, just have to solve the displayport issue (I really hope it is just a bad cable).