Thank you for your feedback! It is the most useful tool I have for improving the game. I cannot read the mind of the thousands of players who play the game. So I rely on people like you who give feedback.
I am also not happy with the kitchen mechanics that I have made. I am currently reworking the drink making aspect of the game to feel more natural than throwing ingredients into a blender.
I agree about there not being a use for three glasses. But this issue should go away when I rework the ordering system. I had initially thought of a quick fix that would give you combo points for giving drinks to customers in a short period of time. This however does not fix the underlying issue of the gameplay loop, which, like you mentioned, is lacking depth.
In the future I will do my best to give clear Visual and Audio indicators.