For instant games, I think C3 changes the files into raw data URIs. In previous versions of the plug-in (1.x.x), raw data URI are not supported by spine-ts 3.8, so it will not work.
However! In the beta version of the plug-in 2.x.x which uses spine-ts 4.0, there may be a chance for this to work, but I have not tested it yet, there may be work to do.
You could try the 2.x.x C3 spine plug-in version, just to see. Otherwise, wait for me to test and figure out if some changes are needed to enable the raw data URI support (I have not developed FB instant games before.)