I have no clue what your salt level is right now.
What was the salt level up to prior to your PWC yesterday? 1 teaspoon per gallon, 2 teaspoons per gallon or some other amount?
When you did the 50% PWC, how much water did you remove? You mention adding 5G back with three tablespoons of salt. Were these level tablespoons or heaping? Please try to stick with teaspoons per gallon so there is no confusion on your end or the readers end. I know a level tablespoon is equal to apx. three teaspoons so you added nine teaspoons to the 5G bucket? Was the bucket filled to the top or 1/2 full or some other level? Without knowing any details, I don't know if you're keeping things at the same level or constantly changing the salt levels.
When you do a PWC, if you remove 3G and the tank had 2 teaspoons per gallon in it, then when you replace the 3G, add 2 teaspoons per gallon to the replacement water so you are keeping the salt level the same.
As far as what caused your fish loss, I don't know since I don't know your salt levels. If you are at only 1 teaspoon per gallon and you followed my earlier directions, then you should not be seeing any fish deaths related to the salt... so I would presume it's the Ich. If you are at three teaspoons per gallon, then how were the fish doing at two teaspoons per gallon? Did you observe the fish for a reasonable amount of time after adding the followup doses of salt to see if they were showing signs of stress related to the salt?
Give us details and post each time you do something... or even before you do something instead of afterwards when it's too late to say no.
Why did you do the 50% PWC yesterday? Unless you are seeing some kind of nitrate issue getting really high or some other issue, there's no need to do a PWC during this treatment. It should only take a week or so and if you did a PWC prior to starting the treatment, you should have been OK through the treatment.