How to setup Syntastic as python3 checker instead of python2 - vim

Question

In MacVim, I save following code as test.py
print "Hello world! python2"
, which is apparently wrong with python3, but
after I run :w to save the file, there is no error message for it,
Following is a part of ~/.vimrc, which is all about Syntastic:
" Syntastic
"" Recommended settings
set statusline+=%#warningmsg#
set statusline+=%{SyntasticStatuslineFlag()}
set statusline+=%*
let g:syntastic_always_populate_loc_list = 1
let g:syntastic_auto_loc_list = 1
let g:syntastic_check_on_open = 1
"" Display checker-name for that error-message
let g:syntastic_aggregate_errors = 1
"" I use the brew to install flake8
let g:syntastic_python_checkers=['flake8', 'python3']
How to make Syntastic detect this type of error as I run test.py in Terminal:
NingGW:Desktop ninggw$ python3 test.py
File "test.py", line 1
print "Hello world! python2"
^
SyntaxError: Missing parentheses in call to 'print'
Following is what :SyntasticInfo said:
Syntastic version: 3.8.0-10 (Vim 800, Darwin, GUI)
Info for filetype: python
Global mode: active
Filetype python is active
The current file will be checked automatically
Available checkers: flake8 python
Currently enabled checker: flake8
Press ENTER or type command to continue

Answer 1

flake8 is a Python package. It uses Python's built-in facilities to parse code, so it accepts syntax for the Python version that it belongs to.
How to install it for your python3 installation depends on how that installation itself was installed - unless you're fine with using pip.

Answer 2

From the FAQ:
4.11. Q. How can I check scripts written for different versions of Python?
A. Install a Python version manager such as virtualenv or pyenv, activate the environment for the relevant version of Python, and install in it the checkers you want to use. Set g:syntastic_python_checkers accordingly in your vimrc, and run Vim from the virtual environment.
If you're starting Vim from a desktop manager rather than from a terminal you might need to write wrapper scripts around your checkers, to activate the virtual environment before running the actual checks. Then you'll need to point the relevant g:syntastic_python_<checker>_exec variables to the wrapper scripts.