If this is wood framing, there will be essentially no improvement in sound isolation by adding the third layer, except for below the resonance of the stud cavity. Most likely not worth it, especially as there are probably numerous flanking paths anyway.
EDIT: Sorry, read too fast. If this is truly a decoupled space with independent stud rows, then yes, adding a 3rd layer to each side would be expected to slight increase performance. However, my point above stands that, given the likelihood of flanking paths, there is little point to adding a 3rd layer.