Re: short forms in reply
Your second and third examples are fine, although #2 might be better as "I already did". The first is very unnatural. I really can't think of a shorter answer than "No thanks, I've already had some/one" as it gives the answer to the question and explains why. Of course "No thanks" is a perfectly acceptable answer but clearly you wanted to show the reason.
Originally Posted by English4everyone
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.