Translation:I would like three lumps of sugar in my milk.
Why is "meine Milch" is in accusative here? It is something for "three pieces to be put INTO my milch". Cannot it be said along the lines of "three pieces (already) in my Milch", which, I suppose, would take Dativ here?
Is it a standard way of saying "I want [some additive] in/on [food/drink]"? Always Akkusativ?
I think you are right about the INTO thing. In 99% of the situations you would say this you would be asking for three sugars to be added to your milk (or coffee, more naturally). Following that logic I would assume that it is standard to use akk. for this but could someone with a better understand confirm or correct me?
here is a link that describes the issue of plurals of nouns of quantity.
„Ich möchte drei Stück Zucker in meine Milch HABEN.“
“Möchte (mögen)” is a modal verb and the sentence should therefore be grammatically completed with a full verb. In this case it is missing because it is rather colloquial and the missing verb “haben” is understood.
The sentence needs the accusative because of the movement. I ask for having sugar put INTO my milk.