I agree with purplekush989, although I'd watch for yellow lower leaves as a sign to start hitting them LIGHTLY. Some say to start after first real set of leaves too so hey, maybe test it on one and observe. Since the soil has nutes it may be better to lean toward safety. Perhaps MG All Purpose @ .5 tsp (the little end of the spoon) to 1 gal and see how they do before upping in in a week or two. If you were to go 1 Tbs like for "outdoor plants" it could be a severe over dose of nutes. 2.0 or 2.5 tsp to a gal once a week would be way later about the max before switching to MG Bloom Booster when you switch the lights/times to flower.

Advanced Nutrients' Nutrient Calculator - Analyze Soil Chemistry and Plant Nutrient Levels for Your Hydroponic or Indoor Garden.

This chart is for another brand. It's nice because you can change the veg, flower, and use the "bar graph" as a picture of the curve to phase in and taper off for the transitions.

Soon you're going to want to get into PH and flushing. Sooner or later, everyone does. Flush once a week is good. Nute once a week is good, some half dose twice a week. Some B-1 vitamin from home depot is good. Super-thrive is crazy shit, I'd get some.

3 plants in one pot is usually not best. YMMV