Sorry, the feature isn't implemented. I didn't even know it was in USFM3.0, actually. That particular example, of course, is only necessary in terms of cell 1 being empty, but presumably it is supposed to fill to the combined width. That's not going to be easy to implement, given how the code tries to divide up space, so, urm, don't hold your breath.
Otherwise... So far we don't distinguish between USFM versions, but try to support everything. (caveat... make that everything which we think is reasonable/possible). Eventually we'll have to start distinguishing between USFM3 and USFM3.1, as the nesting rules differ.