First, note that iOS has a lot of limitations, and you will NOT be able to install pyMC3 or anything else that requires C.
Pythonista comes with numpy matplotlib, and a few others, but anything that requires compiling that is not included cannot be run.
That said, you can install pure python modules, if you first install
See the readme for how to install. This will give you a simplified pip version that works with most pure python modules.