When I initially downloaded and installed LMDE 6, Python3 was also fully installed, and worked out of the box. The method of installation was the standard one, where the distro takes over complete command of your PC. By far, this is the most recommended installation to have.
However, if you find it necessary to do some other kind of installation, for example using a virtual machine, your experience with the Python3 installation might be different. My son did this, and found that certain fundamental components of Python3 were missing. To get it up and running, he typed the following commands, one by one, into the GNOME TERMINAL:
sudo apt-get update
sudo apt-get install python3-tk
sudo apt-get install python3-pil python3-pil.imagetk
Beyond this tip, which may be useful to you or your technician, I cannot offer more advice I am afraid: personally. I have never carried out any installation other than the standard "safe" one.
