you can either add alias to your ~/.bashrc
alias pip=pip3
or add to your $PATH symlink named pip pointing to pip3 binary
(btw, this even though concerning pip isn't really python related question, so you should retag it)
Update: July 2020
If there is no ~/.bashrc in your home directory on macOS, inputting
alias pip=pip3
in your ~/.zprofile file has the same effect,
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