I have an ATI Mobility Radeon 5450 with two monitors connected to a VGA and HDMI ports in a laptop HP Pavillion dv3. I usually work with the two monitors with the laptop screen turned off. In the Catalyst Control Center I can see the three screens but I can select only two. I navigated all the forums mainly in this site but also in others and I couldn't find a clear solution to turn on the laptop screen and work with three separated monitors. It shouldn't be very difficult but my impression is that it is impossible. I read about displayports, external cards with usb, matrox graphics, etc. In the AMD site the specs says "Four independent display controllers" so it should be possible I think. Thank you very much!
It might be due to the constraint of the GPU since its the M version for laptop. I have a laptop with AMD 5000 series card too and I manage to drive max of 2 screen at 1 time only. Maybe you should submit a support ticket to AMD as the Mobility Radeon 5450 is different from the desktop version.
Most people here deal with non-laptop GPU , AMD is your best bet for support on Mobility GPU.