either the battery is prematurely (terminally) ill or theres a constant draw in the vehicle. Charge it up, take off one cable, let it sit that way for you said 3 days right, If it's OK obviously there is a full time draw somewhere in the car. Might put a VOM on amperes scale on the lowest scale(micro or milli amps) & in series with the cable you took off & see if there's current flowing. I'd try the amps scale 1st then the milliamp scale 2st then microamps so you dont fry your meter if it's flowing alot of current.