a quanto pare il problema è qui:
quindi devo mettere un adattatore mdp -> hdmi ma ATTIVO e non passivoYeah... Sounds like you're doing it wrong. One thing... To enable three monitors in Eyefinity mode, one of them MUST be a DisplayPort monitor. And, to be absolutely clear, it MUST actually have a native DisplayPort on the monitor itself. You cannot convert a DVI or HDMI monitor using a cheap passive adapter. If you are trying to do this, you can buy an active adapter, but it will run you about $100. Dell makes a good one that works. Apple also makes one but it does not work well with non-Apple displays (and gets terrible reviews, but most of those come from people using non-Apple displays), and I would avoid it and get the Dell one if you need it.
For the other two monitors, you can use either both DVI or one DVI and one HDMI. This is all due to the timing resources available on the card. Both DVI and HDMI (which are internally extremely similar) require special hardware timers, and the 5x00 series cards have two of them, like most graphics cards. Native DisplayPort does not require external timing signals, and so supporting multiple DisplayPort monitors is "cheaper" (hardware-wise) than using DVI. You can't use one of the passive DisplayPort <> DVI adapters, because the DVI monitor still requires the external timing signal, and it isn't available because both hardware timers are being used by the two other DVI monitors. The active adapters provide this timing signal on that the DVI monitor needs.