every 3 days?!
Changing the water once a week is fine, at least things will be more established that way, enabling the water chemistry to at least be somewhat stable.
Are you use carbon in your filter?
If not, then it will take longer to filter out other impurities (chems) in the water.
Another thing you could do is maybe find out orginially where your fish came from i.e. if you got it at the lfs did their supplier raise the discus in R.O. Water (Reverse Osmosis), or were they raised in regular tap water (de-chlorinated), because if they were raised in R.O. water, then chances are they are weaker, therefore not having a good reaction to your water (which I assume is regular tap water), since R.O. pretty much strips the water.
Another possibility is the de-chlorinator and other possible chemicals you put in the bucket of fresh tap water, which could be causing a "cocktail" therefore stressing the fish.