When are these fruits in season?

Dragon fruit are only grown in tropical areas of the world-- If you are getting it in California, it's imported. I know it grows year-round in areas like Thailand and Vietnam.

As for Mangosteens they are very rare apparently in North America. This is what I found. Due to restrictions on imports, mangosteen is not readily available in certain countries. Although available in Australia, for example, they are still rare in the produce sections of grocery stores in North America and Europe. Following export from its natural growing regions in Southeast Asia, the fresh fruit may be available seasonally in some local markets like those of Chinatowns. Mangosteen and its related products, such as juices and nutritional supplements, are legally imported into the United States, which had an import ban until 2007.

As for Pomegranates this is what I found about them. Pomegranate season typically runs from October to January. Although a native fruit of the Mediterranean region (used for thousands of years), most pomegranates found in US markets come from California's San Joaquin Valley.

Pomegranates definitely seem to come to the market in the very late fall to winter in North America. They are plentiful at Christmas time, and I know this b/c I've decorated with them for Christmas before. When I lived in Greece however, we picked them from the trees and ate them in the late summertime, I think it was. Mangosteens and dragonfruit I unfortunately cannot tell you anything about.

i live in michigan and i usually see dragonfruit at a produce store in the fall-winter

you can get those fruits in Nth Queenland, Australia( cape tribulation )

There in season now