How to tap the full power of a 3-phase circuit
Yes, it is possible to convert a 3-phase AC supply to a DC supply by using a full-wave 3-phase bridge rectifier. Assuming a constant DC load, each phase of the AC supply will be loaded equally.
The resulting DC supply could then be converted to a single phase AC supply using pulse width modulation. Again, given a constant downstream single phase AC load, each phase of the upstream 3-phase AC supply would be loaded equally.