I don't use plants in my C. variegatus and C. elegans setups because, since they're nocturnal and I have an awful lot of setups, I don't use lights. The biggest problem I find with the plants I do use for my many planted enclosures, is finding plants that will actually grow given the light and the temps. For the desert species (C. variegatus), I'd recommend trying aloe or haworthia. You could plant them directly into the substrate, or you could use a variety of succulents in their little pots. You could "hide" the pots with some decorative rocks, being sure that the rocks won't fall down and hurt the geckos if they climb or burrow. The other 2 species aren't really "tropical" though they do like a bit more humidity. You could try pothos for them, which is sometimes the only plant that will grow decently in my planted enclosures. The succulents should work as well.
Aliza