12/12 is not going to give you very much growth, and being an auto, it should be getting 18 hours of light and 6 hours of darkness for proper growth.
Your seedling will not perform or yield much more than 1 gram if you continue with 12/12. Unfortunately it might be too late for this seedling, being 3 weeks old and an auto flower, it should start flowering quite soon. Also, next time you grow, fill your pots right up with soil, this will give much more room for root growth, which then leads to much more plant growth too.
12/12 might be an interesting experimental way of growing extreme sativa photoperiod plants, but is not really a technique that is recommendable in general and definitely not a technique to be used for auto flowering plants - unless you are trying to grow miniature plants with minimal yield.
Hope this helps,........
Organoman.