There seems to be a lot of confusion about POV, probably because there's a lot of conflicting stuff on the Internet, and because writing nerds tend to complicate things.
Third person limited/singular means that the story is told in third person, with a single character's POV. It's like first person, with Jack or Mary taking the place of the "I" character, except that the selected character doesn't talk directly to the reader (at least, not as narrator). So there is no third person limited with three or four (or even two) characters.
When you have several character POVs in a third person story, that's called third person unlimited/multiple. Theoretically, there's no limit to the number of insight characters, but like the practice of switching types of POV in a story, things can get ridiculous.
Third person omniscient is redundant. Omniscient POV doesn't exist in first or second person POV (unless the first person POV is that of God).
Omniscient POV causes the most confusion. Here it's useful to think of POV in terms of permissions. First person gives the writer permission to see into the mind (or through the eyes) of one character, the "I" character. The same is true of third person singular, but with a named character. Third person multiple gives the writer permission to see into the mind of more than one of the characters. Omniscient POV also gives the writer permission to see into the mind of more than one character. In addition, it gives the writer permission to divulge information that the characters are not aware of. The writer can describe scenes when no characters are present.
But here's the critical thing about permissions: Just because you can do something doesn't mean you have to. Just because you've adopted a third person multiple POV doesn't mean you have to provide insight into more than two characters, does it? Permission doesn't equate obligation. So omniscient POV can allow the writer to both tell the story as an all-knowing source, while at the same time revealing the thoughts of the characters. Or not. There are a lot of things to use or abuse. Or confuse, if one loves strict definitions.
So if you decide to utilize omniscient POV to open a story with a description of a valley where no characters are present, a place where none of the story's characters have yet visited, that doesn't mean you have to play the know-it-all throughout the story. There are uses of omniscient POV that do tend to lock the writer in to an aspect of the POV for the duration, such as telling of relationships that your insight characters are unaware of, or considerable revealing of events none of them are witness to, but even so, that doesn't necessarily change the way the writer treats insight into characters. Remember the permissions.
Some confuse objective POV with omniscient. Objective POV can mimic omniscient in scenes such as the valley I mentioned. But with objective POV, there is no permission to employ insight into characters. It's strictly limited to reporting the actions and dialogue of the characters, and the settings that exist in the story. The character's minds are locked up tight, even if their mouths aren't. Just as in real life. Or in most movies.
So the permissions should be observed, but if you want to use an omniscient POV to describe something that requires it--a space station, saturated with radiation and filled with corpses, revolving slowly in space--then do so without feeling the need to work Buck Rogers into the scene. The next scene, where Buck does arrive, will allow you to dip into the heads of our intrepid space cowboy and his sidekick without breaking any rules at all. You can even reveal that both men are wearing the same Wiley Coyote boxer briefs--something that neither knows--if that enhances your story and doesn't stick out as an isolated gimmick.
Happy writing.