When fondant dries it stays kind of soft. Fondant is like play dough. I use fondant for most things but some things fondant just can’t do! Gum paste will dry rock hard so I like to use it for banners, bows,flowers, those kinds of things. Gum paste has the consistency of bubble gum before it dries. It is also good because you can roll it out paper thin. It is traditionally used when making flowers.
Modeling chocolate is more of a clay type medium. It is great for making people, characters. It is a bit stronger if you need something to “stand up.” I also think it tastes the best out of the three. Also because it is like a clay you can smooth out seams with it. Meaning if you made a person you could add arms then smooth out the seams to look like one piece. I hope I have helped answer your question! Let me know if you have any more.