+/ She will have finished her work by 7pm
+/ She will have finished her work before 7pm
Not much difference betwqeen these two- 7 is the limit. The second sounds as if she will have finished the work comfortably bbefore the limit, while the first could go up to the last minute.
+/ She will have finished her work at 7pm
This could be making an assumption now about an action that has finished- I know her timtetable and it's 7-15, so I can say this in answer to a question about her whereabouts.