Does this three engine design have manual reversion in case all hydraulics are lost?
No.
However, you do have four independent systems each driven by its own variable speed pump. Backed up by two Air Turbin Motor (ATM) pumps and two Power Transfer Units (PTU). The ATM's are driven by engine bleed air or APU compressor air. The PTU's mechnically transmit power from one system to another, without fluid transfer. In the event of all three engines fail there is a Ram Air Turbin (RAT) which provides enough power for controled flight.