Finally I found the solution myself, see the Docs:
Just call:
pythonXX -m pip install SomePackage
That would work separately for each version of installed python.
Also, according to the docs, if we want to do the same thing in windows, the command is a bit different:
py -2 -m pip install SomePackage # default Python 2
py -2.7 -m pip install SomePackage # specifically Python 2.7
py -3 -m pip install SomePackage # default Python 3
py -3.4 -m pip install SomePackage # specifically Python 3.4