As for n.1, Are you asking two separate questions? If you are, I would suggest
What wil the distribution of Variable B be? Which is a product of X and Y? or
if your which is a product of X and Y is a statement, I would rephrase the question as follows
Considering/Given that Variable B is the product of X and Y, what will its distribution be?
Then again it sounds like Economics or Maths, neither of which I am an expert of by all means...
As for n.2 you could also say
Let me know when you exit the file
As for n.3, I would prefer
It all depends on the economics (what economics? e.g. the economics we have discussed). Of course the bold part in brackets you may leave out, depending on whether the economics you've referred to may be easily inferred from the context.
Please note, if you decide to stick to your n.3 please amend ITS to IT'S
Hope it helps