In #1522 it was decided to not support 3D compressed textures until someone came up with a strong case for why it is needed.

In our engine we use 3D compressed textures as part of the volumetric cloud system. Naturally these are decently large textures (being 3D) which is where compressed textures can really be beneficial. On desktop BCn compression easily reduces these volumetric textures to a quarter of their uncompressed size, with ASTC 3D block footprints giving even better reductions on mobile. Given memory limitations on many devices (and browsers) we need to take any compression advantages we can get.