what to do if moudle not in pip3 only in pip2?

[snippsat]

*** up until now on all the code I wrote , I allways used python3 only in the command line

Then you have been using old python3 version that comes with OS.
Type python3 in command line and i guess Python 3.5 interpreter start.
Then exit() and type python3.7.
When install with pip3 install modemcmd it will install to Python 3.7(then you most use this version).
Try to install modemcmd again
Then from command line python3.7 ,as i show under here.
Here i use virtual environment that made before, see that both modemcmd and serial works.

>>> from modemcmd import modemcmd
>>> import serial
>>>
>>> modemcmd
<function modemcmd at 0x000001CCDA68D670>

>>> serial.__version__
'3.5'
>>> exit()