One addition to VolvoGirl's excellent answer: don't delay filing the 2018 just because you haven't done the 2016 or 2017 yet. Each year's tax filing processing is largely independent. If you have the data ready to do the 2018 return, then do it, and then work on gathering the other tax information. That is, there is no requirement (once they are late) to do the returns in chronological order. Do whatever you can as soon as you can (because this cuts off interest and sometimes penalties if you owe the IRS).
