I'm not so sure these are very good examples. Monaco is effectively a tiny gated community of France for the super rich that produces little-to-no goods or services. Luxembourg is heavily dependent on support and the existance of the EU (receiving approximately €3,000 per person, the highest in the entire EU). Singapore is probably the best example of a city-state succeeding outside the framework of a larger state, but the size of their public debt is a little frightening for such a small country - and would be slightly less so if it was within a larger body.
Monaco and Luxembourg both are inside the EU and exist very much as a result of cooperation with France (and the Treaty of Utrecht). They both have ceremonial militaries only, and subsist by facilitating tax avoidance for the nearby elites.