Your plans and math don't match reality. 3x3 with a 600w light? Wasteful...
2 gallon pots? For SOG with small plants? Pointless.
Harvest 3 plants every 2 weeks with 12 total? Nah, I'd do more plants and harvest every week instead.
Try 1 gallon pots or less, cram as many as will fit there's no need for space between pots, adjust everything accordingly, veg to about 8-10 inches before flower. You don't even need a gallon, you could use half that and double the number of plants. The limiting factor is not soil you could grow the same size plants with 20 oz soda bottles even, the difference is how often you have to water.
Consider smaller light or more space, 4'x4' would be better with that light. That might not sound like a big difference but it's huge considering you could nearly double the number of plants.
Look at what this guy does with a similar space... http://www.icmag.com/ic/showthread.php?t=88449 Maybe more than you need but it gives you an idea of how an ideal SOG is done compared to what you had planned.