I traded with cartographers for 3 maps (from three different cartographers) of wood land explorers. But now I checked them out and they all lead to the same mansion !?!? they are all the same, all 3 of them ? Is that normal? Do I need to find one and then get a map for the next?
Do I need to find one and then get a map for the next?
Yes. Explorer maps show the nearest, yet undiscovered structure of the given type. And whichever one is "nearest, yet undiscovered" will remain so, until you actually do something to make it "not the nearest" (go somewhere else) or "not undiscovered" (go find it). "Nearest" is based on the location (whether it's of the player or the villager, I'm not sure, but would require very precise locations to make any difference) where the particular map trade was first unlocked.
If I go to the nether, the directions of N S E W correspond to the N S E W of the overworld right? So I CAN travel back home much faster by following a given direction ?
Yes, that's correct. In fact, it's more than just a loose directional correlation. The coordinates (x, y, z) in the overworld correspond exactly to (x/8, y, z/8) in the nether and, likewise, nether coordinates (x, y, z) correspond to (x*8, y, z*8) in the overworld. Note that the vertical "y" value remains unchanged, only the horizontal coordinates are "stretched/compressed" like this. So if you want to go to a particular overworld location of, say (8000,4000) for example, then you can travel to nether(1000,500) and build a portal there which will take you directly (or at least very close to, depending on the overworld terrain at that location) your target destination.