Can a single potentiometer be hooked up to multiple fans to control their speed?
I am looking to hook up a Potentiometer in order to control 4 12VDC Fans.
  I am not sure if that is a good idea...


Well, controlling a fan with a potentiometer directly is a bad idea. First off, your control will be very non-linear; secondly, you will be dissipating a lot of power in your potentiometer, and unless you have a very large (and therefore bulky and expensive) pot, it will fail (most pots are not designed to dissipate lots of power, and the ones that are are huge).

However, you can use the pot to control PWM driver that controls the fan speed efficiently. This is how your computer motherboard controls the fan speed. And once you have that PWM control signal, you can use that PWM splitter you linked to control many fans.

However, you may have an issue: some fans have four wires and some have three. Three wire fans have +/- supply and a tach signal (more on that below), while four wire fans have a PWM input as well. If your fan doesn't have that PWM input, I'm not sure that product you linked will work well. See this answer for more.

You'll need a PWM fan control circuit, or you can buy a pre-built PWM fan controller.


  Also, my fan has a 3 pin input (red, yellow, black). Which wire is negative on this PC fan?
  
  Says "7V sensor" or "tachometric signal" so I'm curious exactly what
  that is for?


Red is positive supply voltage (which can be PWMed for speed control). Black is ground. The yellow wire is a tachometric signal, it outputs pulses that let the driving circuit monitor speed. See this question and the excellent answer for more info.