There are two main types of third-person narration. One is called limited third-person, which, chances are, is what you think of when you think of third-person. It's what I use in most of my writing.
In limited third-person, the narrator isn't all-knowing. The reader gets to know the thoughts of only one POV character at a time. Their attention is directed at only what that one character sees, hears, smells, etc. They only know how other characters are feeling and reaction by listening to their dialogue and guessing by their expressions. To sum it up, if the POV character wouldn't see/hear/know it, then the reader doesn't, either.
But then, there's something else, called omniscient third-person. In this style of narration, the narrator knows all. The reader gets to hear everyone's thoughts, see through everyone's eyes. In a sense, this narrator is like God* and knows every single thing that's going on. This is sometimes referred to as "head-hopping", because the reader is hopping back and forth inside people's heads as the narration progresses.
Omniscient third-person is trickier to do well, though. I personally don't care to read it, because to me it just feels all over the place. To do it, you've got to have a good balance. Some books focus mainly on one character, and throw in another's thoughts every so often. This is disorienting for the reader, and you shouldn't do it. If this is the case, you need to switch POVs completely.
NOTE: Omniscient third-person and having separate, multiple third-person POV characters is not the same thing.
So, here's the thing. If you can't use omniscient third-person well, don't use it at all.
*Except that God's amazing at omniscient writing. In fact, he can pull off omniscient first-person, just because he's...well, God.