As of now edit option is not available, once GSTR-2 is filed. I believe edit option will be available. Regarding GSTR-3B, there will be not penalty.
I think giving wrong turnover will not have any implication except that your turnover may be verified in case you are taken for scrutiny.
In future file your returns correctly. Sincerely I believe edit option should be provided as these are the initial months and not only you 90% people have made mistakes.