Apart from giving strong light, food and water, there is little you can do to accelerate growth beyond the conditions that the plant is living under. There are no "grow faster" magic bullets, however, providing strong lighting is the only way for a pant to grow to its full potential, providing quality food and water are also part of this , as well as as much room as possible for root development. Going to an 18/6 light schedule will actually give you a healthier plant, 21 hours of light is too much in my opinion, cannabis can only grow so much in any 24 hour period, and growth tops out at about 18 hours, meaning anything lighting longer than 18 hours is basically a waste of electricity. Plants do actually need to "sleep" and 3 hours is just not enough for the plant to maintain a healthy metabolism.
Hope this helps,..... Organoman.