Ok Thank you .
Diopters are easy to use and cheap, but you will not get as sharp results as with a dedicated macro lens. Since autofocus is difficult to use with macros an old Micro-Nikkor 60/2.8D is doable and quite cheap used compared to the G version which *will* autofocus on the D3000. Tamron and Sigma macro lenses work well, and the current models have built-in focus motors except for the Sigma 50. The Nikkor 60G has a nice bokeh at longer distances and does double duty as a nice portrait lens.
The D3000 is not ideal for macro photography since the pentamirror construction steals some light in the finder, but it can be used.
Like UntamedSpirit said above, the 40 mm lenses are difficult to use because of the short working distance.
My favourite macro lens is a bit too cumbersome on a D3000 because it will not meter, so you would need to take test pictures or at least check the histogram for every picture. It is an *old* Micro -Nikkor, the 105/4 K lens. They were made from 1975 to 1977. Unlike it's more modern cousins it does not change focal length when focused close, which is an advantage.