There is no substitute for scientific discovery.
That's a quote from...me.
I have a chlorine test kit. If the water is .5 ppm or more it is safe to drink. Anything else and you are just guessing. 3 weeks may be a sewer, or it may be just as good as the day you put it in. It depends on how clean the tank was to start, and how good the water was when you put in.
We have a charcoal filter under the kitchen sink to take out the chlorine flavor at that point.
We have well water at the house, but I chlorinate it for safety when it goes in the tank. We and our dogs have been drinking out of our FW tank since 2007.