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

or

if yourwhich is a product of X and Yis 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 youexitthe file

As for n.3, I would prefer

It all depends ontheeconomics (what economics? e.g. the economicswe 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

Shan