Sorry, I don't quite understand the question of how is 3ml water enough? English isn't my mother tongue can you specify?
Sadly, I don't have the confidence in my skills to give you a proper assessment of your plan.
However, to my knowledge, both Phenobarbital and Chloral Hydrate could lose some potency if exposed for a longer time to H2O. Otherwise they should be fine
I am assuming you want to create knockout drops with CH, use morphine for pain relieve and PhB to kick it, right? Sounds good enough, Ig.