All of them are correct , but the most spoken is "when I am finished" and the most written is "when I finish it"

You can use any combination of those words, but personally I would use "I am finished" because it flows better. Also, it seems the most likely when I say it aloud, and you can do it if you are not sure to check for fluency.

All three are correct. However, only 'when I finish it' definitely means after you finish the report. With the other two, this meaning is possible, but they could also mean after you finish another task you are currently doing.

I'll see you at the park after school.

Looking to the first phrase, when I (finish it) part is correct. It refers to report.