As I understand it a "flush" is where you water your plants with 3x the amount of water as you have in pot volume....so a 3 gallon pot would be flushed with 9 gallons water. Is this accurate? And is it neccessary to do this? Can't you just stop feeding the plant nutes and use plain water for the last couple of weeks but water with normal amounts and let the medium flush over time or do you have to do a huge flush like I described above before harvesting? Thanks.
You are describing two different types of flushing. To flush before harvest you do as you describe cutting all of the nutrients and feeding it plain water for about 2 weeks. The other flush you describe is to clean the soil of salt build ups and other things you do not wish to have in your soil any longer. O almost forgot it isn't necessary to flush before harvest but your buds will taste bad and your plants will still have nutrients in them.
The first flush you discribed I see soil growers do every other month so that there is no nute lock-out, and salt build up. The 2nd on is to rid the plant of chemicals that will big the buds a bad smoke, and harsh taste.