If I understand it right, technically this is actually the fourth game, and there is a Metal Slug X that is an enhanced version of Metal Slug 2 which uses the engine that this game uses (shoot, wish i had played that maybe instead last night! Zombie mode is fun!) Don't get me wrong, this game is great, it looks even better than the first two with a really rich pallete that looks well beyond 16 bit at times
Game starts out stunning looking from the very first stage. wow!

The gameplay isn't really changed. Everything in this game is way harder. Some boss fights are true quarter stealing nonsense where you can barely hit them or angles are inconvenient or something.. Game theme is mixed up and not as overall sticking to a central idea as the first two games.
what is cool tho... is turning into a zombie, which is what this game introduces! when undead you can fire a bloody 'godzilla atomic breath' that kills stuff real good.
A great Metal Slug but it is not my favorite even with its engine enhancements, additional weapons and vehciles, and zombie mode.