The README says to download the .zip file and then evaluate Platform.systemExtensionDir and place the contents of the unzipped file there.
On my system, that's ~/.local/share/SuperCollider/Extensions, but the contents of the zip file itself appears to have a bit of redundancy in directory names. To whit:
$ unzip sc3-plugins-3.13.0-Linux-x64.zip
...
$ find . -maxdepth 4 -type d
.
./share
./share/SuperCollider
./share/SuperCollider/Extensions
./share/SuperCollider/Extensions/SC3plugins
./lib
./lib/SuperCollider
./lib/SuperCollider/plugins
Following the directions as written means I will end up with a directory:
~/.local/share/SuperCollider/Extensions/share/SuperCollider/Extensions/SC3plugins
It seems saner to move the unzipped /lib/SuperCollider to ~/.local/lib/ and the unzipped /share/SuperCollider/Extensions/SC3plugins to ~/.local/share/SuperCollider/Extensions/...