Black Council
There are three senior black council members, who wield mortal wizard powers, built three time missile barges separately.
Logical evidence:
Building three barges separately and simultaneously minimize chance of discovery. Having one being build all three seem an ineligant and slow strategy. Harry "the help" had to be rushed out to assist with this surprise attack.
Speculative evidence:
Mac tells Harry there are three. Possibly, he means three dark wizards, not three outsiders. Why send only one Walker to demonreach, if there are three Walkers.