My biggest concern with an absence of 3 weeks is either the water level getting too low and burning up the pump, or debris clogging up the skimmer, stopping the water flow, and burning up the pump--everything else can be solved with little problem. If you use a leafnet or mesh cover, it would stop the debris from getting into the skimmer, but not stop the water level from getting too low. Usually when I leave, my "pool friend" comes by daily to make sure everything is okay before she turns on the pump, but then again we have HUGE evaporation rates.
If you're using trichlor tabs as your only source of chlorine, I'm betting your CYA is very high, and you are very likely to come home to green water.....you might consider adding a dose of Polyquat 60, then shocking the pool before you leave. Your shock level, though, is gonna be based on your CYA level (see the "best guess chlorine chart" link in my sig for more explanation and the chart.
My concern is that if something goes wrong with the pool, leaving it "wrong" for a week is likely to have expensive consequences!
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