By this time tomorrow or at this time tomorrow or tomorrow at this time

It depends what you mean. Examples 1 and 2 mean the same thing, and are both grammatically correct. They both convey that, at this specific time tomorrow, you will be in the process of watching. There is also a suggestion that you will start watching it at this specific time.

As @mplungjan indicates, there is a difference in example 3. it is also grammatically correct, but the phrase by this time tomorrow means something will happen between now and the same time tomorrow. In your example, the watching could start at any time between now and then. You may be beginning the movie at this time tomorrow or you may be completing the movie or anywhere in between.

The phrase will be watching conveys that the activity will still be ongoing at this time tomorrow, not completed. To convey completion by this time tomorrow, you would say

I will have watched a movie by this time tomorrow.