The FN P90 is a Premium SMG available for purchase for $1.99 or 200 SAS-BUCKS in the mobile version of SAS: Zombie Assault 3. It has a higher damage and reload speed than the PP19 Bizon, and uses standard SMG ammo.
This gun is lowly recommended to buy unless you use SAS BUCKS from challenges, because it is the most inaccurate gun in the game, including shotguns, which is a way to scale it down due to its cheapness and monster firepower. It performs the best in apocalypse mode, where it does not require much accuracy given zombies come from all directions.
Due to its cheapness for purchase and cheap ammo to use while providing with monstrous firepower, at expense of accuracy, it is useful for farming money and/or bucks quickly.