AMD Radeon 6900 3 monitors problem

Bros,

 

I am using the AMD Radeon 6900,

I would like to extend the 3-screen display, that is 5760X1080 resolution

But I have been able to do only two screens, 3840X1080 resolution.

 

My connection is such a way, I would like to ask why can not be extended to show three screens, thank you

Screen 1: HDMI --- HDMI

Screen 2: HDMI --- HDMI --- MINI DISPLAYPORT

Screen 3: VGA --- VGA