When i use a mirror modifier, i want the duplicate to be made 3 blender units in the X axis, but it actually makes it -3 Blender units in the X axis. To summarize this up, the modifier is duplicating in the wrong direction. How can i correct this?
The mirror modifier works according to the object’s origin - go into edit mode of the object to move it around without effecting the origin, and hence without effecting the mirror
What about when it is centered, I keep getting a seem right down the center that only gets worse the more I do near it.
I don’t really know what you’re talking about 
Screenshot?
Also try using the mirror modifier ‘Object’ box thing - so the mirror will reflect onto that other object (empty would be best)