sorry for this being a boring explanation but i figured a few people would benifit from this.
okay so to unlock vinyl groups on horizon 3 we need to use Vantage, and have our own created layer group and a downloaded one on horizon 3
Once we have that we want to go to the horizon 3 save where you will be presented with a mess of files

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you will see two different types of layer groups ones like LayerGroup_291836 and LayerGroup_0000_89637343566.
The ones with _0000_ are your own created ones and ones without are downloaded ones.

i suggest making a scrap layer group as you can delete it after.
what you will need to do is go to your own created layer group and press the plus button next to it which will display these files

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so you have

c_group
header
thumb.png

you will need to click on header ( make sure you click on the text to download your unlocked header
(also this doesnt matter which one you use as long as its your own one)
this next step will be painful if you have loads of downloaded groups
so to get the locked Layergroup_000000 you are looking for you will need to save the thumb.png from each LayerGroup_000000
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BUT MAKE SURE YOU ALWAYS OVERWRITE THE LAST ONE YOU SAVED SO YOU DON'T END UP WITH HUNDREDS OF FILES

once you have found the one you wish to unlock simply click the upload button on the right of header.
and upload your unlocked header that you saved earlier
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once the uploading circle goes away its all done.

Be aware the layer group will now be named the same as the one you took the header from.

Locked:
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Unlocked:
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it should be the first layer group displayed when you go to My vinyl groups.
im not a coder or programer so i cant make a program to do this but anyone else can feel free to do so please just give created where its due. Also you cannot just change the text in the header it will corrupt and crash the game so you have to swap the locked header with an unlocked one.

EDIT: If you are in game when you do this you must quit the game then relaunch it for this to take effect. you can also use the same method on tunes and on designs

i beleive i am the first to do this. If anyone needs help my xb1 GT is RNY and im also named RNY on the discord chat for Vantage
Many thanks and enjoy