On Windows pip3 should be in the Scripts path of your Python installation:
I can install pip3.4 with Python 3.4 on CentOS 6.5. But I used the same method and failed to install pip3.4 on CentOS 7. Python3.4 is OK though.
pip3 should be installed when you install python. if you didn't select the add to path, then you can find where the pip3 located and add it to path manually or you can reinstall.
python - how to install Pip3 on windows 10? - Stack Overflow
Whether you use pip or pip3, it will be equivalent to python3 -m pip as mentioned in jakub's answer. Also, given that Python 2.7 is already EOL (which means you will most likely work with Python 3) and that pip install -ing things onto the system packages should be avoided, then a virtual environment would be helpful here.
which is the same message I get when I type just pip. Does it mean that in previous versions, things were different, and now pip and pip3 can be used interchangeably? If so, and for the sake of argument, how can I install packages for Python 2 instead of Python 3?
pip or pip3 to install packages for Python 3? - Stack Overflow
The other answers are technically correct, but are a little unclear on why Python has both pip3 and python3 -m pip: Using pip3 to globally install a package can be ambiguous if you have multiple Python installations on your machine.
How can I install Python's pip3 on my Mac? - Stack Overflow