Downgrade core from 4.0 to 3.x

Linux Mint 17.
I installed VSCode and PIO many months ago, but didn’t use it. I don’t remember how I installed it. About a month ago, I finally started using it, but I’m having some strange things happen, and my co-developers can’t help me because they’re on 3.x of the PIO Core, and somehow I’m on 4.0.0a23. I’ve Googled for over an hour trying to figure out how to either delete PIO Core entirely, so I can start over, or just downgrade to the latest 3.x version.
pip uninstall platformio and sudo -H pip uninstall platformio both yield the same response: Skipping platformio as it is not installed.

I uninstalled the PlatformIO Extension in VSCode, but when I reinstalled it, the Core is still 4.0.0a23.

What can I do to get off of 4.0 and on 3.x?

Nevermind. I FINALLY found it. For anyone else who needs it:

In PlatformIO, in a terminal window:

pip install -U "platformio<4"