Declan, strictly speaking 'third person' isn't a point of view, it's just grammar. In a narrative told in first person, the narrator is the character who calls themself 'I'. If you're writing in third person, then there's still an
implied narrator:
someone has to know, and be be telling the reader, that 'Emily got home from the mine, and John poured her a drink.'
You then have to decide between two possibilities:
1) that narrator only knows about what's going on inside the head of one character, and can only see and know what that character can see. This is usually called 'third person limited' or sometimes 'limited subjective', in the sense that what's told and explained and understood is how that character would see/explain/understand it - which may not be true.
If you limit the pov to one character at a time, you can still switch between different, limited PoVs: the usual advice, at least until you're confident handling PoV, is only to do that at chapter or some other major break. But the narrative is always rooted in a single PoV.
2) that narrator knows what's going on inside the heads of more than one character and at more than one place, can switch at will,
and knows what will happen. This is usually called 'third person omniscient' or sometimes 'knowledgeable', and many writers consider it the natural, most fluent and grown-up way to write. Many writing teachers will tell you not to do it, for reasons I don't understand.
If you have an omniscient narrator, then obviously they can tell you what a character doesn't see, or doesn't know, or doesn't understand.
The key is to decide whic you're doing, and do it consistently through the book. What doesn't work is to take the reader deep and long-term inside one head, and then suddenly write a sentence as the hat-check girl sees it, or tell us 'little did he know that it was his last day on earth.'
Some good, basic advice here (tho' I don't agree with all of it)
http://helpineedapublisher.blogspot.com/2010/01/points-of-view.html
and here:
http://helpineedapublisher.blogspot.com/2010/02/more-points-of-view-says-who-says-i.html
Emma
<Added>And this might help with the point that Rosy highlighted, about how deep you are (or aren't) inside a character's head:
http://emmadarwin.typepad.com/thisitchofwriting/psychic-distance-what-it-is-and-how-to-use-it.html