3 weeks is not a long time and there is not a lot you can do to subtaintially change the thesis at this stage. So focus on making sure your references are correct and taut. Then focus on intro and conclusion. I had to take this approach and it worked.
Take care of yourself, Regular breaks, but not too many, and no deleting of material to change it. Saying that, I did delete half a cjhapter the night before submission and didn't replace it. It worked better that way. In general stick with what you have - deleting only the dire and not the "hmmm couldbe better". Good luck.