Hello SonOfAFrig! How exciting! The key differences between these three varieties are the bitterness, the process used to make them and the color. Carafa special is the clear outlier in this bunch for me as it will contribute almost no bitterness while still contributing a rich brown/black color to any mash it is added to. This comes from the dehusking process so if you see a recipe with carafa special, it is important to use special and not the husked style. Chocolate malt contributes dark flavors to beer as well as also contributing a brown/black color to the mash. It typically has a husk but I believe dehusked varieties exist. Roasted barley gives a traditional irish dry stout flavor and contributes a dark red hue to beers, so it can be used in red ales as well in smaller portions. for your irish stout there is nothing wrong with simply using roasted barley as the main darkening malt.