Combining multiple AC/DC adapters
Yes, devices of the same voltage can be run from the same supply.   

For this you would need a (preferably decent quality regulated) 12V supply that is capable of sourcing more than the total (max) current draw of all your devices combined. If you have other devices at e.g. 9V you would need a supply with a 9V and 12V output.
The required info re current draw is usually written on the device somewhere, or in it's manual. You can also use the rating info on the adapter it came with as a guide.

For instance (for 12V devices) if device 1 draws 100mA max, device 2 200mA max and device 3 500mA max, you need a supply capable of supplying at least 800mA @ 12V.  

For high current draws, a SMPS will be preferable to a simple linear supply. These will be much smaller/lighter in comparison.