If you are going to run a 3kw system, and are using Enphase M215 inverters, you will only need to run a 14/3 wire plus your #6 ground wire for the racking (assuming your code requires it and AFAIK they all do). Each inverter will output a max of 0.9Aac so with a 250w panel you will have 12 x .9A x 1.25 (safety factor) = 13.5A. You will need to go to a 15A double pole breaker.
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