Filing taxes for 2015 through 2019

I'm helping a family member file their taxes for years 2015 through 2019 so they can claim the stimulus payments on this years taxes. There's a rather lengthy back story that I wont get into aside from the fact that their mom was filing taxes for them but today looking at their transcripts online that never happened. The mother is long gone. 

I'm not well versed on filing previous year taxes that are 3+ years old. 

 

If in 2015 a refund was due to from the taxes do I still need to file 2015 and 2016? Since they are 3 years old I know a cant be claimed but should I still file them? 

Basically I want to file everything that I need to by law even if a refund cant be claimed so there are no problems down the road and so that we can claim the stimulus payments on this years taxes so she gets them.

But I don't want to spend time on filing a years taxes if I don't have to. 

I have this persons wage and income transcripts ready to go.  I just need to hunt down some w2s because the Federal EIN is not complete on the transcripts just the last 4 digits. 

 

Any help or knowledge would be appreciated