Need Help with Light and Humidity

Hi I was wondering if you guys could help me out. I have 3 Grand daddy Purp plants and they take up about 5'x3'. Is a 400watt HPS enough for all 3 plants. They have been in 12/12 for a couple weeks. 2 are def flowering and one is lagging behind a little bit but is still flowering just slower than the others really. The light is about 16 inches from the top of the center plant and obviously a little further from the others. Maybe about 20 for the others. I also have a humidity issue. its like 30% humidity. Is that ok during the flowering period? Will Heat stress happen? I also have a fan blowing a little bit between the HPS and the plants. Here are some pics. Let me know what u guys think. Thanks again

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