I know how to set up a fan on a Pi 3.
But can I attach this fan to the GPIO under the Pi by soldering it and will it affect the PiTFT that is actually attached to the Pi GPIO on top? And could the fan still be controlled with a Python script?
You are certainly not the first person to have asked about using a cooling fan with the Pi.
I can understand why people think they need to actively cool their Pi. From a desktop PC user's perspective the idea of running a CPU without any cooling seems bonkers.
I have extensively used various models of Pi over the past couple of years and not once I have been concerned about the Pi overheating. Firstly, the SoC which is the main chip on the board, where things like the CPU are located has a thermal limit of 85°C. This means if the Pi approaches this temperature it reduces the performance and the temperature decreases.
Having used a Pi 3 in applications such as a media center, not even watching a film at 1080p could get the CPU temperature anywhere near that. Also a cooling fan is rather redundant anyway unless you plan to use heat sinks. Heat sinks allow the thermal energy to be stored up over a large surface area. This makes dissipating the heat energy far more effective than just blowing some air on the SoC.
I've had a look at the case you are referring to and I'm struggling to imagine fitting even a tiny cooling fan in it. Yes you could solder it to the GPIO pads on the underside of the Pi, do I recommend it, well... It's one of those were if you have to ask it might not be a good idea. You do risk bricking your Pi in the process and I imagine you are going to void any warranties in the process.
I'm also not sure if the GPIO is going to be to handle both the requirements of the case and the fan simultaneously. I suggest if you are concerned about temperatures to keep an eye on them, if they are not going anywhere near 85°C then the cooling fan is really superfluous.