The first and the third sentences are grammatically possible. The second makes no sense. The third construction is the one we'd be most likely to use, although 'finished' is the verb we'd probably use - 'I'll show you when I've finished it'.

you could also say, I will show you when it is finished. Possibly an American English speaker might be more likely to say "I will show you when it is done".

1 and 3 are OK but 2 is not. There is no real difference in meaning between 1 and 3 but the present simple and present perfect do have different usages in other contexts.

Hi Adel, Micheal is quite correct 1 and 3 are fine, however as native speakers we would much more be likely to say "I will show you when I have finished it".