Of those three boats, I would suggest the Mystere 4.3. It is very versatile, because it comes with main, jib and spinnaker, so you can sail with main only; main and jib; main and spinnaker; or main, jib and spinnaker.

Long Island Sound is usually light air in the summer, so you might have more fun, too, with the 4.3 because of the spinnaker.

Don't even think about the Nacra 6.0 if you are going to singlehand. It is a very powerful boat; and there is no way you could right it by yourself if it capsized.