9 plants, 3 gallons each/ 27 gallons total, in a 3x3 tent, you're going to have humidity issues in mid- late flower after stretch.
I grow in a 3x3 and I have 9 plants in my tent right now, but they are in 1 gallon pots. My humidity stays around 55-60%.
Wait until those plants get a few nodes and some good growth on them to start giving them substantial amounts of nutrients.
I'm sure you could give them a light 5-10% strength dosage of nutrients if you want.
That light & tent combination is badass. I will be following this grow.