The last date to file belated return for AY 2017-18 was 31st March 2018. You can file the return now against any notice received for filing the return.
No. You can only file against the notice. If you have received any notice for filing the return for AY 17-18, you can file the return. you cannot file belated return now.
You can file your return only for Financial year 2017-18 now. Assessment year 2017-18 return is time barred and cannot be filed online. If there is genuine delay, you may approach income tax dept for solution.