Tropico Reloaded contains the original Tropico (which is stellar) and its expansion pack, as well as Tropico 2: Pirate Island which isn't so good. Tropico 3 is a three-dee remake of the first Tropico with some added features and improvements. If you had to choose between Tropico Reloaded and Tropico 3, I'd say get the newer Tropico 3. It has everything in the original but isn't nearly so dated.
All that said, Tropico 4 comes out next month. T4 updates T3 with a new engine, new interface, and some new features. And you get Tropico Reloaded as a preorder bonus. There is a demo on steam that actually gives you quite a lot of freedom. From what I've seen, Tropico 4 seems to be the best in the series. I'd grab that if I were you, El Preisdente.