If your bike is a presently a double, then it was designed for that purpose only. The front derailleur cage of a double is shaped quite differently from that of a triple, the 'middle ring' won't have ramps or pins, but it will all likely at least work. These innovations did not always exist. Cheap thumbie type friction shifters won't have too much difficulty adapting to a triple, but fancier stuff might. If only because there won't be any labeling for the new gear. OCD is a *itch. So, as you say, you will find out.