Where did you download the fixture types? From the FixtureShare?
If yes, from the grandMA2 (gray-yellow) part of the share or the grandMA3 (gray-white) part of the share? There is a switch in the top right corner before logging into the share, where you can switch the modes.
I assume you downloaded grandMA2 fixture types:
Copy them into the grandMA2 folder structure in your usb drive: /gma2/library.
From there you can import them into grandMA2.
grandMA2 onPC and grandMA3 in Mode2 allow you then to convert the show (at the moment only the patch) within the backup menu to grandMA3 (Save patch as grandMA3 x.y.z).
When you load the converted show in grandMA3 then you can export the converted fixture types as grandMA3 user fixture types and use them in other grandMA3 shows.
There is also the possibility that you downloaded GDTF fixture files from the GDTF share. You don't have to unpack these files.
Put them on your usb drive into this folder: /grandMA3/shared/lib_fixture_types/gdtf.
From there you should be able to import the files while adding new fixtures in the patch. Just switch from Internal drive to the usb stick in the top right corner of the Add fixture dialog and select the GDTF tab.
In addition you can also create your own grandMA3 fixture types within the grandMA3 software. Therefore, go to fixture types (top left corner) in the patch menu.