Pip3 is the Python3 version of pip.
If you use pip, then only the python2.7 version will be installed. You have to use pip3 for it to be installed on Python3. So to install packages in python3, you should use pip3.
NOTE:- Its not necessary that pip will install in python 2.7, if python2 is absent then pip will do it in python3. The above statement was if you have both the version of python installed.