I had a similar question a couple of years ago on here and learned a lot from other members. I have since picked up even more. The lens you have is one of a multiple used in cameras like this. They were common in photo studios producing carte de visite and gem photos in the second half of the nineteenth century. A tin type could be exposed with multiple views or at the same time (one view) and then cut up.

As you may be able to see from the camera referenced, the threaded end of the barrel was toward the sitter, not the film plane, as the lens board was a sheet of brass with threaded holes and the lens barrels projecting back toward the plate. I have found some of these lenses with the elements reversed by previous owners so that the threaded end could be screwed into a flange and the lens project out toward the sitter. This catalogue, from the very informative antiquecameras.net site, shows the Darlot Gem lenses. It also has a great little bit at the end of the Darlot section on the proper positioning of elements in Darlot lenses.

I'm not completely sure of the differences between series 3 and the other series made by Darlot. It is a reference to focal length and coverage, I believe, but I don't have the exact relationship clear in my head. The one I had was a 4 and covered about a 3 1/2 inch image circle. Here is an image shot with it on 4x5 film.