we currently have formats for 7-inch, 10-inch and 12-inch vinyl, but not yet 3-inch.

examples of 3-inch vinyl releases:

https://musicbrainz.org/release/b628835b-8243-40db-808e-d1420cdb6090

https://musicbrainz.org/release/b85b90e7-9291-4a9f-9617-58db82433c26

https://musicbrainz.org/release/5f98c8e8-0ca3-44b2-bd03-9a827825b897

https://musicbrainz.org/release/c7b01615-30d4-48bd-978e-7c89fcda655b

https://musicbrainz.org/release/a4a9d31d-2e5c-45ac-bfe5-cdf16175b589

https://musicbrainz.org/release/73fe2406-790e-46d1-9cc6-6253aefbb840

https://musicbrainz.org/release/31e5f9f1-9144-4434-86f2-7b9a618f3d68

https://musicbrainz.org/release/cb4a9f31-a7c4-4b32-af87-7b983da8d548

https://musicbrainz.org/release/b27ed828-1ea9-4877-942b-b93f937ee890

there are even series comprised entirely of 3" vinyl.

https://musicbrainz.org/series/18b97ad7-8cf1-4b1f-8181-e72e6cbf20bd

https://musicbrainz.org/series/fced91e0-6575-468f-bc8b-0eeb12ca77c5