It is too early.
-- MODs at this stage are not supported.
Running phpBB3 live is also not supported until RC1.
The best method to convert from yabb to phpBB3, would be to use the convert yabb to phpBB2 using the existing yabb->phpBB2 converter, then convert that phpBB2 to phpBB3.
You won’t be able to use the yabb->phpBB2 converter to convert directly to phpBB3.
The structure of phpBB3 is too different.