There are currently 5 maps for SAS 3.
In order of unlock, these are:
The first two maps have an XP bonus, Farmhouse giving +40% XP and Airbase +20% XP. This is presumably to compensate for the much lower zombie spawn limit. Note that there is no money bonus for either, so you will most likely end up profitting less from these.
Farmhouse
The starting map for the game. It is the smallest map in the game. Gives a +40% XP bonus to all players.
Tips
There are 8 barricades in total on this map making it liable for the player to attempt to keep at least one room fully barricaded at all times, and camping in that spot. For more experienced players (and higher level players in particular) circling through the house and making a Nazi zombie style rape train is the best option.
Airbase
Airbase is the second map map, unlocked after finishing a match of Onslaught on Farmhouse. It is set in a partially destroyed airbase. A plane wreck can be seen breaking through one of the walls, and a maze of totalled cars to move from one area to the next. There are 10 barricades on this map.
Tips
For most players (especially in singleplayer) it is suggested to move to the upper area, as there is much more space to maneuver. Another technique in multiplayer, should only be used when in a team of experienced, trusted and/or competent players is to be used. It will work best with 3 others. This technique is to stay in the room at the very bottom of the map, the one with barricades to the east, south and west. The player who has the best gun, or is the most skilled should face out into the rest of the map while the others cover a window each. This strategy can also be used in singleplayer, replacing the 3 barricade campers with turrets, you know, if being a lone-wolf (loner) makes you feel badass.