A new feature in Metal Slug 3 is the branching path system; in most missions, there are forking paths from which the player must choose one, each with their own obstacles, and each of varying length and difficulty. All paths eventually lead to the same boss battle, but the player may have different equipment, and may have rescued more or fewer POW's depending on their choice of path.
Vehicles
Several new vehicles were introduced into the game:
The "Slug Driller" can be found in the underground part of Mission 4. The drill is capable of mowing down every enemy in front of it and can also be extended. Additionally, the Driller can increase its height, making the player unreachable to ground enemies.
The "Slug Mariner" gives added firepower when underwater. It fires high-mass rounds which sink to the bottom, as well as torpedoes.
The "Elephant Slug" is an elephant strapped with a Vulcan cannon. By picking up a battery or a chili pepper, the elephant can fire a bolt of lightning or a fireball from its trunk, respectively.
The "Ostrich Slug" is similar to the "Camel Slug" in Metal Slug 2/Metal Slug X, although it is faster, can jump farther, and can turn around.
The "LV Rebel Armor" is a rebel-produced vehicle that can be captured and used by the player. It comes equipped with a heavy machine gun (though in Mission 5 it comes equipped with a flame shot instead), which is interchangeable with other handheld weapons. It also has a grenade launcher, a melee claw attack, and jump jets. The second player can stand on top of the Rebel Armor, and can receive a boost.
The "Slug Copter" appears alongside the "Slug Flyer" in the airborne portion of the Final Mission. Its Vulcan cannon can fire in all directions, unlike the Flyer. It is also armed with gravity bombs.
The "Astro Slug" is used jointly by the players and the Rebel forces. Booster rockets carry it into space, where the player can make use of its weaponry. Like the Rebel Armor, the Astro Slug can use infantry weapons through ports on its sides. It fires rockets as a secondary weapon.
Character transformation
As with Metal Slug 2/Metal Slug X, the player can transform into various different states. In addition to the mummy and fat forms from the previous game, other forms include:
When doused with infectious material, the player becomes a zombie. While medication is available to reverse the effect, the zombified player can make use of a devastating (but slow) vomit attack that covers most of the forward area. The player cannot crouch or jump high, but is immune to attacks from enemies (except zombified enemies). If a zombified player comes into contact with infectious material, they will die.
Underwater, the player equips a Scuba set with a bubble helmet. Rather than grenades, the player uses a bomb-launching system with the ordnance affected by gravity.
When flying, should the player lose their vehicle, they rely on a backup jet pack that can fire heat seeking missiles.
When in space, if the player loses the Astro Slug, they must depend on a jet pack and a breathing apparatus. Explosives are contained in the pack, and when fired they cause a massive fireball.
Plot
Several years ago, the evil General Morden was foiled in his attempted coup d'état against the worlds' governments by the Peregrine Falcon Strike Force. After forming an alliance with an alien race in an attempt to stage another coup, Morden was betrayed and taken prisoner by his new found allies. His rebels troops then formed an ad hoc alliance with the Strike Force, and ultimately, the aliens were defeated, although Morden himself was apparently killed.
Instrumental in defeating Morden during the first coup were Cpt. Marco Rossi and Lt. Tarma Roving. Rossi (now a Major) and Roving (now a Captain) also led the fight against the Morden during the second coup, this time joined by two members of the Intelligence Agency's Special Ops Squad S.P.A.R.R.O.W.S.; Sgt. Eri Kasamoto and Sgt. 1st Class Fiolina Germi.
Several years have passed since that time, and Morden is officially listed as missing by his surviving followers. Determined to wipe out every remnant of Morden's powerbase, the army send Rossi and Roving to destroy all remaining rebel strongholds, one by one. During the fighting, however, Rossi and Roving come to the conclusion that the enemy is too well organised, and perhaps Morden is not as dead as was initially thought. Meanwhile, the S.P.A.R.R.O.W.S. come across a series of strange events which lead the army to conclude that the aliens with whom Morden once allied himself have returned.
The Peregrine Falcon Strike Force and S.P.A.R.R.O.W.S. are once again united, and sent to defeat this new threat. After various battles against Morden's forces, the heroes are shocked to face off against Morden himself. However, after defeating him, it is revealed that it was really an alien in disguise, and the real Morden has (once again) been taken prisoner by the aliens. The aliens then abduct the player character and leave Earth. At this point, another character takes the player character's place. Once again, an ad hoc alliance is formed between the Strike Force and Morden's troops so as to save their captured comrades. The rebels launch an armada of rocket ships to attack the alien mother ship, Rugname. After a long battle through the ship's interior, the Rugname starts to collapse due to the amount of damage it has sustained. After destroying the ship's core, the Strike Force must fight their way through armies of clones of their captured teammate, some of whom have turned into zombies. Eventually, both the captured member of the Strike Force and Morden are freed. As they escape, however, they are confronted by the aliens' leader, Rootmars. A battle ensues in Earth's atmosphere, which is won by the Strike Force, who leave Rootmars' body in the ocean. Upon seeing Morden and his men celebrating, the player character throws his/her weapon into the water in disgust.
