Just wondering how I should choose between the three. Are some better for certain things? Is fondant or marzipan better to cover a cake with or is it just preference? Thans.

I haven't tried it, but I've read that marzipan takes a long time to dry. It's not going to get as hard as gumpaste. And I've read it's tastier, but again, I haven't tried.

Marzipan is very expensive and generally rather heavy. It's used to make the surface of fruit cakes smooth enough to accept fondant. It's not generally used to cover American style/sponge cakes.

Fondant is the medium used over buttercream or ganache when covering American style cakes.

Gum paste (flower paste in some other countries) is used for making flowers, figures, and molded elements. It can be mixed 50/50 with fondant in order to slow drying time a bit and prevent brittleness and cracking when making figures.

HTH
Rae