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
http://GitHub.com/ywangd/stash
See the readme for how to install. This will give you a simplified pip version that works with most pure python modules.