BobKamman
Level 15

IRS response when you ask about the 2016 return:  "Yes, we received that envelope.  No, there was not a 2016 return inside -- just 2017 and 2018."

I would mail each year in a separate envelope, and use tracking (or certified, if you must) for 2016.  When more than one year is sent in the same package, they tend to be stapled together and only might be processed.