No, de-dupe perhaps is a poor term but you reference that the 3 of you have many of the same songs. If you merge by simply exporting and importing "all" music, you'll have upwards of 3 copies of the same song in the main library. So, "de-dupe" means locating all the duplicates and purging them so you end up with 1 copy in the main library. Apple Music has a File (menu), Library, Show Duplicate Items option to help identify duplicates if you want to de-duplicate manually.
There's lots of ways to merge libraries. However, if all 3 libraries are mostly the same, maybe you have a way to get each person to identify the unique songs in THEIR libraries. Then 2 can export their unique songs to drag and drop into the main library. This could be easy if the libraries really don't have many differences and each of the 2 have a pretty good sense of the unique songs they've added to their own.
If everyone's libraries started from copying a core library to 3 computers, you can sort by date to see what has been added since that core library was copied to each. The list of newer songs since added is where ALL of the potential for new, unique songs to have been added to each. This MIGHT be an easy way to quickly find ALL of the unique songs on each of the 2 computers that will not be the main Mac used for home sharing. Export the unique ones then drop them into the main one and there should be no duplicates and all music would be in one place.
If the three of you can't do either very well, there are tools out there that may be able to help. For example,
this one is about merging 2 libraries (while not also bringing in the duplicate files). I haven't used it and there's probably others like it. But it looks like it will bring the unique songs in one library into the chosen "master" library. So you use it for 2 libraries and then use it again to pull library #3 unique songs into the "master" library. Then turn on home sharing in the master library so everyone has access to all of the songs in the main library... which should then be ALL unique songs the 3 of you have.
Again, I don't know if there is a way to preserve individual playlists on library #2 and library #3. Maybe there is a tool for that somewhere? I haven't dealt with this kind of thing for a few years. But if you three have mostly the same taste and mostly the same playlists, this may be no big deal. Truly unique playlists may need to be recreated from scratch but each can look back at the list on their original Macs to help them recreate it on the main Mac.
Once everyone is confident they have their music and playlists as they want them on the main Mac, they could delete the duplicates on the other computers, freeing up substantial space for all that duplicate music. Then, everyone is fully using home sharing for access to the combined music library.
It is likely that the shared library and unique playlists on each device work just fine. If so, once each person can connect to the home share (master), they create playlists on their own devices to basically re-create how it was when they were all working with their own copy of the library on their own device. This MIGHT be the grudge work in this merger, especially if either of the other two actually have a good number of unique playlists to re-create.
One more thing: think this through. For example, if the main library is on a laptop that sometimes goes out with someone, the other two will NOT have access to the music library when the main one is not available. So generally, this is best done with a dedicated desktop that will always be home and thus available to everyone.
Lastly, the "shared folder" concept is not really the way for this. It's not really about folders with how Apple apps store media. You let Apple Music do the folder managing and let the main Mac's Apple Music app manage all of the media. Home sharing is working with Apple Music, not with some isolated folder. While you could probably put all of the music on some kind of home network drive, it's easy for network issues to create disconnects and then "Apple Music can't find file" errors can pop up. The easy way is the way Apple conceived with home sharing from one master Apple Music library. I wouldn't stray from that.
READ UP on all of this to get your head around these concepts as fully as possible. Back up your individual libraries so you can "go back" if you try it this way and don't like it. In my home, we all are perfectly happy with home sharing from one central desktop Mac always on and thus always available to everyone (including AppleTVs, iDevices, Sonos speakers, etc).
