“for my wife “?

you have to be an American citizen at the time her green card was approved to be eligible for the 3 years waiting time

I would assume that when your wife arrived in the US, she had an immigration visa stamped on her passport, and shortly after, she received her Green Card in the mail. Your wife has been a resident more than 3 years since she received her Green card. From May 2018 until now, it is less than three years, so you have your citizenship for about 2 years and 9 months. So, I believe your wife would be eligible to apply once you complete the three years. Also, believe that you can not apply 90 days before the three years since you, the husband, should have your citizenship for at least three year on the time of submitting her application