David, much research turned up no information on applying this years tax refund to past due amounts, probably because of the fact that I haven't filed meaning there is no tax obligation at the moment until I file those missing years. Seems to me the best course of action would be file this years return, and send in that same money plus return for one of the years tax obligation i.e. 2016 or 2017. After much deliberation on my part and seeing no one else respond to e-filing vs. mail filing, seems e-filing would be the best option in not tripping up the AUR machine, would you agree?