Medcol

People are answering your question without enough information from you.  One tax return COULD be enough, if you have a legitimate reason for not filing the other two.  Otherwise, you need information from 3, but are only required to send a copy of the latest.

You need taxes from past 3 years. Not just one, I believe. 

She needs to send at least 1 year.  And if it will be okay or not depends, the NVC doesn't make that call,  the CO at the interview does. 

Then she sends a copy of the amended tax return for 2017 showing a correction to married (jointly or separately it doesn't matter) to NVC and you take a copy to the interview.

Not an immigration issue, but it should be fixed with the IRS as Single was not a proper filing status.

If you two were married at the time she filed the tax, it should be married.