Here is the set-up process for my wife to share my bookmarks.
ln -sf target_file/foldername symlink_filename ....... Base Command
In her system, in a terminal:
mv -i bookmarks.html bookmarksBAK.html
ln -sf /home/denise/Fred/fred/.mozilla/firefox/79gmvcic.default/bookmarks.html bookmarks.html
This allows her to see and write to my bookmarks when she opens her Firefox. She is actually using my bookmark file through the symlink. Both of our /home directories are read/write for the other. Otherwise we would have permission problems. You shouldn't have that problem since you used the same username.
Now... in the Tomboy thing. Maybe I am not understanding something, so correct me if I am getting it wrong. You have three installs, with Tomboy and a config folder in each. You want to use the same config folder with all three installs. I am assuming that regardless of which program puts a note in the config folder the other install programs can see and use it. This could be the fly in the ointment.
Assuming we can do this, we want to rename the config folders in Mint 4 & 5 so they aren't functional, but won't be lost. Make sure Tomboy isn't running. Let's make a symlink to the config folder we will be using in Ubuntu. I am assuming it is called .tomboy
mv -i .tomboy .tomboyBAK
ln -sf /home/richard/BuntuHome/richard/.tomboy .tomboy
You should now have a symlink in the Mint install you did this in called .tomboy. If we click on it we should see the inside of the .tomboy folder on Ubuntu. If we do the same thing on the other Mint install we should see the same thing when we click on that symlink too. We have only one .tomboy folder but we can see the inside of it by clicking on .tomboy on any one of the three installs.
If you get to this point then you need to test and see if all three installs can read and write to that one folder and see the other notes that were put in by other installs. That will determine if this idea will work. The Tomboy program may or may not recognize the work of the other programs. I just don't know, as I said before. I am hoping for the best.