I found the answer the worked for me on my Pixel 2 XL and I assume anyone running the latest Android update:
1) You may as well forget MP3 files. The new OS wants OGG or M4P files.
2) I had to download/install a converted. I used VLC (make sure you get it from Videolan.org).
3) I converted my custom MP3 ringtones to OGG files and saved them on my computer in a folder.
4) I connected my Pixel and allowed to be used for "File" access from the phones pull down notifications after connecting to my computer.
5) I copied the OGG files from my computer using file manager to the Pixel (Internal Storage\Ringtones and/or Internal Storage/Notification tones depending on what you need - I copied mine to both).
Normally when you "import" ringtones this way, you have to restart your phone. In this case, I did not have to do that. As soon as I opened up Sounds and tried to assign a ringtone to an app, the tones were available in the My Sounds section of the new Sound 2.0 user interface in the latest update.