An FC drop from 20 ppm to 3 ppm over 3 weeks is a loss of 8.6% per day. Between the solar cover and the cloudy/rainy days, there was little UV from sunlight getting through. So the main chlorine loss was from chlorine reactions which are dependent on water temperature. Actual loss depends on the organics in the water and what the chlorine can react with (cover, filter, etc.) and there is the slow chlorine reaction with CYA itself. At 88ºF in my own pool, there can be 15% loss per day with no sun, but this drops to 2.5% per day at 50ºF. I'm guessing your water temp is below 73ºF which is where I'd expect the loss you are seeing if you had no UV at all (so the water temp might be lower than that and the rest of the loss is from the little UV getting through).