Selecting text with 3d Touch is not just pressing harder. First off, you have to press hard on the keyboard to get the trackpad – so now in this mode, you can move the cursor to your desired point. To select text, you would think that all you have to do is press HARDER. Well, press all you want, it will not select text even if you are pressing REALLY HARD.
The solution is to actually lighten your pressure without taking your finger off, then gently jab/press into the screen. Now it will select text. It takes a bit of practice and is not like the “test” of 3d touch in the settings app, where you clearly do press harder to get other functions.