Elixir des Merveilles is Eau des Merveilles with an addition of more orange and a gourmand chocolate note - so the original has no gourmand / chocolate note and a more fizzy/bubbly orange, less candied. IMO, if you like the Elixir, you will like the Eau. If you can, test Parfum des Merveilles, an even more intense version of the Eau, with a slightly boozy note added (although its very pricey).
Concentree d'Orange Verte is nothing like any of the Merveilles. It is a cologne/eaux type scent with delicate citrus top notes, a bit sturdier (and synthetic) than the original Eau d'Orange Verte and nice green/sheer woods. Perfect for dousing yourself with, in hot weather.
The Merveilles are much more about ambergris, woods, orange, etc...
All three are wonderful scents, and I think they are all good blind buys.