using pip3 to call pip

[David Epstein]

How do I arrange for my shell command
pip3
to give the pip associated with my /opt/local/bin/python3?

I tried to do this using "port select", but that failed, as you can see 
below. Can someone give some explanation of the meaning of the the error 
message "The specified group 'pip3' does not exist"? What does "group" 
mean in this context?

Macintosh-2:~% echo $path
. /Users/dbae/bin /opt/local/bin /opt/local/sbin 
/usr/local/texlive/2016/bin/universal-darwin 
/usr/local/texlive/2016/bin/x86_64-darwin . /Users/dbae/bin 
/opt/local/bin /opt/local/sbin 
/usr/local/texlive/2016/bin/universal-darwin 
/usr/local/texlive/2016/bin/x86_64-darwin /usr/bin /bin /usr/sbin /sbin 
/usr/local/bin /opt/X11/bin /Library/TeX/texbin
Macintosh-2:~% which python3
/opt/local/bin/python3
Macintosh-2:~% ls -l /opt/local/bin/python3
lrwxr-xr-x  1 root  wheel  24  5 Sep 20:50 /opt/local/bin/python3@ -> 
/opt/local/bin/python3.6
Macintosh-2:~% which pip
/opt/local/bin/pip
Macintosh-2:~% which pip3
/usr/local/bin/pip3
Macintosh-2:~% sudo port select pip3 py36-pip
Password:
Selecting 'py36-pip' for 'pip3' failed: The specified group 'pip3' does 
not exist.
Macintosh-2:~% ls -l /opt/local/bin/pip
lrwxr-xr-x  1 root  wheel  67 11 Sep 10:34 /opt/local/bin/pip@ -> 
/opt/local/Library/Frameworks/Python.framework/Versions/3.6/bin/pip

I seem to lack certain elementary knowledge about the port options.

Thanks
David

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