I placed a very thin ply of tissue paper, slightly larger than the fan itself, by the fan and watched it get pulled right onto the fan and stay there until the machine was turned off.
The two larger fans are obviously on intake, as they have enough suction to keep regular paper stuck to them.
I don't recommend anyone doing this for extended lengths of time, as this will only cause your system to overheat. But for this exercise, it surely proved that, on my 8790 at least, all three fans take air in.
I'm no thermal engineer, but I certainly wouldn't think having a fan on exhaust close to a fan on intake would be a wise design - you'd be circulating your exhaust right back in. Unless we're talking about turbochargers, I'd think havin them all on intake, with the heat exhausted out the back away from the fans would be a better design.