After the excitement of Metal Slug X, Metal Slug 3 returns to being very plain. There are two unused areas and a few warps, but otherwise it's just a basic level select.
Amusingly, if you warp straight to an area where a player is kidnapped, without having done the kidnapping scene just yet, Marco is
automatically chosen, even if you're already
Marco. Even more
strange, the zombie clones are
replaced with Eri, and Eri pilots the
UFO while fighting Rootmarks.
Huge thanks to candy76041820 for the name translations.
Metal Slug 3 lacks interesting test levels. And has a whopping two unused areas.
Andy the Wizard is an empty Mission 2 with no music, and it ends somewhere around the area with the Girida-Os. There's a Type-R Metal Slug at the beginning, and using the Slug Attack to destroy it will respawn a new one, cycling through the different variants of Metal Slug: front and backward facing, regular or Type-R, etc. Thanks to Gutsybird for this discovery!
たぐち is the area in Mission 1 with the barrels of flying piranhas, but there’s no scrolling and the only object there is a Rebel Armour. Again, there’s no music and you parachute in, strangely.
にしの is the same area as FOR 2, but instead of a Rebel Armour, there’s two giant crabs there. After killing them, you can scroll right and find two more, and once they’re vanquished, there’s nothing left to do there.
ふじさわ かんごく is the area where you rescue General Morden from the aliens, but it stops scrolling where the machine he’s trapped in would be, and the only objects are those sinking tanks you find in Mission 3. Odd.
ヤマモト タイラー is the same area as FOR 2 and 3, but there’s no objects anywhere at all, you can scroll all the way to the end, and the boss music plays, making it sound like there’d be something of interest here.
MEE HER appears to be the same as the area you see in FOR 5 and more, but it has the objects you find when you go there in the actual game. There are some minor differences, like finding an ordinary prisoner instead of Uka, getting access to a Mobile Satellite, and finding that same woman from the ending of Metal Slug 1, Satiko Suzuki, near the prisoner. You can’t interact with her, though.
ありた ひみつこうじょう is the same as the warehouse area in Mission 3, except there seem to be more prisoners there.
なかつか まよなか の ほうこう is simply an empty Mission 2. Boring.
ぎりだ is Mission 2 again, but now has a Slug Flyer, a Metal Slug, and a Type-R Metal Slug to choose from at the beginning. Otherwise, it’s empty up until the area where you find the Girida-Os, where a number of them come to attack you and you can scroll no more.
アンミラ is the area in Mission 5 where you first find the clones, but the ending music plays.
There's only one "true" unused stage in Metal Slug 3, the rest are just those pointless "start halfway through the level" ones. I'm skipping the latter.
The only true unused stage, this is similar, if not exactly like the first part of Mission 3 in Metal Slug 1. There are only one type of zombie here, and no items or anything, other than the lifts (which aren't seen anywhere else in the game) and the teleportation pod at the top.
And this is where said pod leads to, near the Girida-Os.