Wait, it's been 3 years since they split? Is that right?

No.
Since they broke up, probably only a few months have passed, at most a year. The 3 years were mentioned because that made the audience realize that it’s been more than 3 years since Mincheol last knew where Haesoo was working (in other words, he stopped asking). If you look at the previous chapters, from the very beginning, everyone knows that Haesoo currently works in 3 places: the BBQ restaurant, the flower shop, and the motel (which she recently quit to spend more time with Taeha). But she had already mentioned that she had other jobs, and apparently the last time Mincheol knew where she worked was at the place he just visited. This shows that it’s been over 3 years since he stopped asking Haesoo where she was working or how things were going (he stopped caring about her), because we all know that right now he doesn’t even know that Haesoo has been working part-time in 3 places.