In Cubism 3 SDK and later versions, the embedded data formats of Cubism 2.1 such as .moc and .mtn cannot be used as-is.

However, Cubism 3 and later versions of the editor can read Cubism 2.1 data formats such as .cmox and .canx and then convert them to .cmo3 and .can3.
Therefore, it is possible to support the newer versions of Cubism SDK by outputting in .moc3 or .motion3.json format after such conversion.
See the Cubism Editor manual for conversion instructions and details.

How to convert Cubism 2.1 format data to Cubism 3 or later format.