Working 12V ups circuit?
Presumably you can find a power supply capable of delivering 12V at 3A to the two devices? Plenty of power supplies like this on ebay etc..

Use a relay powered from AC - when the AC drops out the relay contact feeds your load from the battery. When the AC returns, the relay contact swaps back to feeding power to the load from the 12V power supply.

A slight refinement is to have a schottky diode connected between battery and load (in effect across the relay contacts) - the benefit of this is to always guarantee that as the contact switches there will be about 11.5 volts to power your load. Also recommended is a large ish hold up capacitor across your load (maybe 1000 uF).