I had a 400 hps and 3 plants in almost the same setup as you and I also had a lot of flower drop at first. I was able to resolve the problem by putting the light on a digital timer that had 36 presets. It allowed me to turn on the light 50 minutes and off 10 minutes every hour for 10 hours. I read that the HPS light can get too hot even with the fan for the plant and overheat it causing frequent flower drops. I also only gave them light at night when the temps where a lot lower to help prevent the plant from over heating plus it's cheaper to run the light after 10:00pm. This year i'm going with 2 cool tubes that I picked up for this grow season since the tube is fully enclosed it suppose to remove over 90% of the heat from the light and allow you to get a full 1000 watt hps withing 16" of your plants!