You cannot use Eyefinity with that card...
There's a model of HD5450 that is able to run it but looks like you dont have that card... To be able to use eyefinity (3monitors) your card must have a DisplayPort connector, using it will let you add a 3rd monitor.
As I can see, your card have DVI, VGA and HDMI ports, with that configutartion you can only connect 2 monitors at time...
Sorry but thats how Eyefinity works, only with DisplayPort.