So what does the "Never" condition actually do?
I don't recall seeing it in any Maps so fill me in...

A never condition simply means that this trigger will never fire. It is primarily for your map making purposes. Like you could use them to create blank triggers w/comments. Or if you are testing your map and don't want your trigger to activate, just add a never condition to it rather than disabling everything (don't forget to delete it though once through).

Thats what i thought it was for, i was hoping that someone would say that it means the actions followed would never happen, even by another trigger
Like if you never wanted a location to ever be on your Marine at anytime... Hmm i wonder how you could do that?
NEW QUESTION: How would you make it so that location x would never center on your first marine at anywhere, but center on your second marine at anywhere

If you never want a location to ever be on your Marine at anytime, then just... don't use the "move location" action in that way. But your question is too vague. Like does location x constantly move around by centering itself on one unit? various units? Or is it a nonmoving location? Or do you want this location and your marine to act like magnets of the same charge, where they seem to repel off each other or one will move away when the other tries to come closer?

Ok sorry about the vagueness, i want a location to keep moving around the map centering on Marines owned by player 6.
But i want the location to keep moving to different Marines instead of just sticking to one forever without giving the marines to another player

Ok well first of all, do note that when your triggers detect units on a map, they do it in a specific order. They go from left to right, bottom to top. So if you have a 3x3 set of marines placed somewhere and have your map kill one marine at a time. It'll first kill the unit at the bottom left, then mid left, top left, and then goes to the next row killing mid bottom, middle, mid top, etc.

With that said, if player 6 has a bunch of marines on the map and you simply have a trigger which centers location x on marine owned by player 6 at wherever the marines are, it'll only center it on the left most (and bottom most if two or more units are equally positioned leftwards). If your marines are junkyard dogged, the location will only move to a different unit when it's current centered-on-unit is no longer the left and bottom most unit. But again, this location is only limited to units at the left and bottom-ish area.

If you don't want to use the grid system by giving marines to another player, then what you could do is implement a "touch system." Where initially you set Location X on any unit on the map, have this occur only at the startup or w/e. And then make a trigger with an action reading "Center Location X on men owned by w/e player at Location X." And if the unit with the location centered on it touches another passing by unit, the location will start targeting the other unit. But remember the detection system, it'll only work depending on the relative positioning between the current unit and the new potential units currently touching Location X

At first, I will quickly answer Bol's question:
If you want to center on the 3rd marine that is found in a location you have to give or move 2 marines away first. Then center on the marine, and give/move the other 2 marines back. There's not really another way.

Now on to how the game determines what is the 1st, 2nd, etc. unit in a location. What MillenniumArmy said, is not entirely correct.

First sc checks the x coordinate of the units in question. Lower x-values (leftmost) will be chosen first. (So far MA was correct)
But if 2 or more units have the same x coordinate sc checks their UnitID, higher IDs first.
Think about it: Checking the y coordinate could fail for flying or burrowed units.

You may ask now how is the UnitID assigned? There's several rules for for different cases for it:
1) Preplaced units:
- If you're using Staredit, or X-tra Editor the UnitID is based on the y coordinate. Lower y coordinate => Lower UnitID. That is the reason why those editors cannot create perfect stacks of units (like the 50 mineral patches of fastest maps). And that is probably where MA's misinformation is coming from.
- If you're using ScmDraft 2 or Starforge however, UnitID is based upon order of placement. Units placed first have lower UnitIDs.
2) Units created in game:
Units that are created in game apply to the same rule as ScmDraft 2 and Starforge: Units created first have lower UnitIDs.
Now keep in mind the creation pattern of units. They're like spiraling counterclockwise around the center. That means units are created like this (ideally):

qponm
redcl
sfabk
tghij
uvwxy
Now look at the leftmost column: qrstu
The units are created from top to bottom => bottommost units get detected first. That is another reason why the incorrect y-coordinate rule is so widely spread.

And incase you want to know what happens if you create 25 marines with 1 action, honestly I don't know. But I think they're also spiralled around the center, but within a single trigger loop.