The starting point is the wall wart / Power supply. It supplies 12 V (if it is a wall wart probably to a 2.5mm barrel jack like this one:
(although you will probably need the UK version AC plug not US as this particular wall wart is). The barrel connector on the end plugs in to a socket such as this one:
http://www.bgmicro.com/PWR1168.aspx
and you can solder the power wires from the screens to the tabs of the socket. The arduino wants the same type of plug as the wall wart ends in so you would need to get a plug (of the correct size because there are a number of different sizes) that comes out to wires and solder them in parallel to the coax socket that connects to the wall wart. In the end all that's happening is that three positive wires (2 from the screens and one from the arduino) are connecting to the positive end of the wall wart's plug and the 3 grounds are connecting to the negative end of the wall wart, you are just using the socket to split the one connection from the wall wart in to three to go to the three devices in to whatever connector (bare wire, or a barrel connector in the case of the arduno) that they want.
Peter