You need to give your numbers for a more deifnative answer, but your shock amount is based on your CYA level. With that being said most will agree that with a vinyl liner you shouldnt go much above a chlorine level of 15ppm. In your case that 3 gallons of 6% bleach. You will need to maintain that level until the pool is clear. Might have to add bleach throughout the day to maintain the levels.
John