Eyefinity is about making multiple screens appear to be a single screen. It's not required to use multiple screens normally, where the desktop is extended onto additional monitors, but each screen is separate. To put it another way, with an Eyefinity screen, maximizing a window would make it fill all monitors. With an extended desktop, the maximized window would only fill the screen it's on.
The main use case for Eyefinity would be to make three monitors look like one monitor while gaming. The integrated graphics of a 2400G are not powerful enough to handle that. Beyond that, you generally can only plug two displays into a motherboard supporting APU's.