As the title implies, I'm using version 1.12 Vanilla (until they update optifine) And am trying to recreate the warp pad from the popular cartoon "Steven Universe" by making a 4x4 diamond block square in the ground in a house and my general idea to make it teleport people to the desired location was to use '/testfor @a[x=190,y=62,z=355]' in an area and then send a signal to another command block that would teleport the person who stood there to xyz. I tried a number of different variations but didn't know how to set up the underground redstone and for some reason it seemed to always find me even when I was underground and about 5 blocks away (I was opening the command block to see the output) Any help appreciated.
The problem is that those coordinates aren't static. Your command block doesn't test if there's an entity at that location (speaking of which: I'd use @a instead because you'd only be able to teleport players anyway), but this commands tests for the distance to that position. This confused me as well first time.
Best way to overcome this is to combine the thing with a radius: testfor @a[x=190,y=62,z=355,r=1].
I also wouldn't bother using redstone to signal another command block. I'm assuming you're using a repeating command block (purple)? Just add a conditional chain command block which performs the teleport command and you should be fine. See my screenshot for a rough indication of what I did (horrible build though).
Still... I think you may be better off using pressure plates which in their turn trigger a command block, because this setup will gobble up resources. Those command blocks need to constantly check if a player is present and that will have some affect on performance. Minimal when you only got a few scattered around but if you plan on making a whole network then I could see this having an affect.