Question

My entire growth cycle is 3 months (clone to harvest) and I intend to use good soil (like Pro Mix BX) in 3 gal pots for each plant. Theoretically, this should be enough soil to provide for all nutritional needs, correct? 

Answer

As long as you keep track of the ph levels and the moisture levels of your containers they should be good enough, although if you use bigger containers you will get bigger, healthier plants that will produce more buds and more thc.