Parameters are off, water changes could be making things worse.
The recommended dose is 5ml per 60g so 1ml per 3g is 3 x recommendedAre you sure that the 2 older shrimp actually died? Did you see their milky white carcasses? Sometimes they "act dead" when they are molting.
Also, how much is "3 to 4 drops" of excel in mL? And what is the size of your tank? Excel is can be shrimp hostile if dosed in moderate quantities. I always have ~5% or so of my population die off from a 1mL Excel to 3 gal dosing.