There's a relatively easy way to do this, if the 3100 has a panorama mode. If it does, then your first shot should be of the brightest light. This will set both shutter speed and aperature. Then take the rest of the shots. Ta da!
I've used this technique with my Nikon 5200 while my 5700 is in the shop. It works just as well as using manual settings, except you don't have to fool with finding the correct manual settings for the first picture.
Credit for coming up with this approach goes to tvodrd! /ubbthreads/images/graemlins/grin.gif
For those with full manual cameras, it's best to set exposure from the brightest light. This will mean the others are underexposed, but that gives more meaningful pictures than a bunch of overexposed images, which will result if exposure is not based on the brightest light.
Also, I recommend verifying that you have white balance set correctly and avoid any post-processing. I suggest the later since changing the sharpening via NikonView or Adobe's un-sharp mask, or changing the contrast, brightness, tint, etc, is really modifying what you're trying to capture - the light's actual beam. I also leave in-camera sharpening set to auto. Nikon's in-camera algorithm does a decent job, but errs on the conservative side. All of these suggestions are based on wanting to capture the most objective pictures possible. If you want the images to look their best, giving up some of their usefulness in regards to comparisons, ignore these suggestions. Of course, the usual YMMV applies. These suggestions are generalizations and don't always apply.
One last tip. If your 3100 has trouble focusing, pick up a $10 laser pointer. I've taken images with my 5700 (with no low-light focus assist) late at night under a new moon and it works very well. Just put the laser's beam in the center of the light's beam, half-press the shutter to set the exposure values, turn the laser off, and fully depress the shutter. This little trick works just as well as most of the consumer cameras with low-light assist, at least until you get into the DSLR class of cameras. With my 5700, I usually get focus lock in less than 2 seconds using this technique.
Hope this helps!