The leaves of my 3 small plants are drooping like ...

Brrrrr! 59f at night is a bit cold for a just-starting-to-flower plant. I think the drooping is due to cold. 72f day and 68f night is the lower end of ok in my book. Substrate like to be 22-24c I find. Make your solution temp 22-24c as well or you'll be cooling the roots and slowing development. Put them on something warm/hot. You can use paving slabs (wood does not retain heat very well). Put some slaps in the sun and some underneath your pots. When it gets dark, the slabs that where sunbathing all day, put under you pots to act as heaters for a while. When your media gets warmer, you should see everything pick up, literally and figuratively.

Hi there! Droopy leaves can come from overwatering, and this might result in root rot. Roots are saturated with water and the plants can't absorb nutrients nor oxygen anymore. Are your pots punctured at the bottom, do they have holes so the drain can get out? I think the plastic sheet you put onto the top of your pots will not help soil to breath and dry, but will dramatically increase fungus and bugs invasion risks. On the other hand, you're talking about droopy leaves, but what I see on your last pictures are yellow leaves dying falling down : this is a sign of Nitrogen deficiency. Bloom fertilizers tend to have less nitrogen than phosphorus and potassium, and our favorite plant still need Nitrogen during the first weeks of flowering, when the stretch happens. I did not find any N traces in your diary, when was your last N intake? You may need to give her a few waterings with N and slowly decrease N intakes until she reach her final height and put all its energy into bud production. Hope this helps, I'll be around, keep us up-to-date and happy growing!