The above shots are taken from Metal Slug X, which was developed as an improvement for Metal Slug 2 while Metal Slug 3 was in planning.

Yes, Metal Slug X has all the sprites from Metal Slug 3, minus some backgrounds. Metal Slug X also had backgrounds from Metal Slug 1 and 2. To check that the Slugs and backgrounds that are not in Metal Slug X, but are in other games of the series, play the game on the developer mode using debug console and enter the debug mode and run some test missions.