you do not need a 3 way switch for 12v applications. buy a normal switch, and hook them both up just as you would with 1 switch. when you get both +12v wires to the light assembly, tee everything together (12v from switch A, 12v from switch B, and the + lead on your light). if you have multiple bulbs, all the +12v wires need to be daisy chained together.
***NOTE - make sure the switches you use are able to handle the current the light is drawing. the light bulb should give its current rating in A (amps), as well as the switch(s). if you have 10 3amp bulbs, you need to either buy switches rated for 30 or more amps, or you need to use a relay (which is a whole nother wiring explanation)
the cheap $5 switches you find and stores like napa are not rated for much current. a good place to look for switches is an RV dealer. most lighting in campers is all 12v, and they are rated for the current draw of multiple lights (still be sure to check the ratings of each switch to make sure it will work for you)