Difference-inDifference model with Discontinued Treatment
You presumably should be using all 3 years for the data. Given that you had the appropriate conditions for diff-in-diff (parallel trends), and you're looking at a standard case of the treatment only being relevant in treated periods (no lagged effects), then the additional data should not hurt your estimates.  Additional years of untreated data will help you more appropriately measure the de-facto difference between the two groups, and therefore more precisely measure the effect treatment.
Take a look at panel data and difference-in-differences application - you add fixed effects for all $t$ and all $i$, and create a DiD variable for the the interaction of the treatment and control variable.  It doesn't, under ideal conditions, matter what the timing of treatment is.  If you do have a reason to believe there are lagged effects you should, of course, account for that with lags/leads of treatment, which I believe the other answer is concerned about.